Investing 101 (Part 4): Asset Allocation by Age — The Simple Version

Investing 101 series (4 parts): Part 1: Why investing isn't optional — the math → Part 2: What an index fund actually is → Part 3: Which account to open first — 401(k), Roth, HSA → Part 4: Asset allocation by age — the simple version ← you're here

If you've made it through Parts 1, 2, and 3 of this series, you've already done about 80% of the investing work most people ever need to do. You know why investing matters, what to buy (a low-cost index fund), and which account to put it in.

The last question — the one Part 4 covers — is the mix: how much in stocks vs. how much in bonds, and how that mix should shift as you age.

This is the part of investing most people overthink. The honest, evidence-based answer is much shorter than the internet suggests. One rule of thumb, one shortcut for people who want zero ongoing decisions, and one allocation table for people who want slightly more control. After that, the investing decision is done — for years.

What "asset allocation" actually means

Asset allocation is just the split between the major categories of investments in your portfolio. For 95% of people, that means two categories: stocks and bonds.

• Stocks (the index funds from Part 2): higher long-term returns — historically about 10% nominal / ~7% after inflation — but much higher volatility. A 30-40% drop in a single year is possible. A 50%+ drop has happened twice in the last 25 years (2008 and 2020) and recovered each time.
• Bonds (bond index funds like BND, AGG, or FXNAX at ~0.03% expense): lower long-term returns — historically about 4-5% nominal — but much lower volatility. A 10-15% drop in a bad year is unusual; bonds mostly grind out steady, small gains.

Mixing the two lets you dial in how much volatility you're willing to tolerate. A young investor with 35 years until retirement can ride out any short-term drop — so they should own mostly stocks, where the long-term returns are highest. A 65-year-old who needs to withdraw 4% of the portfolio every year has no such luxury — a 40% stock drop right at retirement could force them to sell at a loss. So they hold more bonds, which cushion the swings.

Everything else — international stocks, REITs, gold, crypto — is optional fine-tuning around this core decision. Get the stock/bond split right and you've handled the load-bearing part.

Why your age matters: time horizon and volatility tolerance

Two things change as you get older, and both push your allocation toward bonds.

1. Time horizon shrinks. At 30, your money has 35+ years to ride out market crashes before you need to spend it. Every historical 30-year window in the U.S. stock market has been positive — even windows that started just before crashes. So short-term volatility doesn't really matter for a 30-year-old.

At 65, your time horizon is as short as a few months for the portion you'll spend this year, and 30 years for the portion you'll spend in late retirement. The blend means part of your portfolio still wants to grow (stocks) and part needs to be available without a 30% haircut (bonds and cash).

2. Sequence-of-returns risk gets dangerous near retirement. This is the under-appreciated concept that shapes most retirement planning. If the market drops 40% before you retire, you have years to recover. If it drops 40% the year you retire, you're now forced to sell stocks at the bottom to fund living expenses — locking in losses that compound badly over the rest of retirement.

Bonds protect against this. A 60/40 portfolio at age 65 means even a brutal stock crash only hits 60% of your money, and the 40% in bonds can fund your next few years of spending while the stock side recovers.

The rule of thumb: 110 minus your age

The simplest, most-used allocation rule:

Percentage in stocks = 110 minus your age.

So at 25, you'd hold 85% stocks / 15% bonds. At 50, 60% / 40%. At 70, 40% / 60%. Walked out fully:

Age	Stocks	Bonds
25	85%	15%
30	80%	20%
40	70%	30%
50	60%	40%
60	50%	50%
65	45%	55%
70	40%	60%

A few caveats on the rule:

• The older "100 minus your age" version is too conservative for most people now. Lifespans are longer than they were when that rule was first written; most retirees today need 25-35 years of retirement spending, which requires more stock growth than 100-minus-age provides.
• A more aggressive "120 minus your age" version is defensible if you have a high risk tolerance, a stable income stream into retirement (pension, rental property), or a partner whose portfolio is more conservative.
• None of these are exact prescriptions — they're starting points. Off by 5-10% in either direction barely matters over a 30-year horizon.

The point of the rule is to give you a reasonable, defensible target without spending hours analyzing your "risk profile" or filling out questionnaires.

The truly simple answer: a target-date fund

If you don't want to make any of these decisions — and there's a strong argument that most people shouldn't — there's a one-fund solution that handles asset allocation, age-based rebalancing, and international exposure automatically:

A target-date retirement fund.

You pick a fund based on the year you'll roughly retire (typically age 65). The fund holds a mix of stock and bond index funds appropriate for that timeline, and automatically shifts the mix toward bonds as you get older. You buy it once, set up monthly contributions, and never have to think about allocation again.

Examples at the major brokerages:

• Vanguard Target Retirement 2060 (VTTSX) — for someone retiring around 2060. About 0.08% expense ratio. Currently ~90% stocks / 10% bonds; gradually shifts toward 50/50 by the target date.
• Fidelity Freedom Index 2060 (FDKLX) — equivalent at Fidelity. About 0.12% expense ratio.
• Schwab Target 2060 Index (SWYNX) — equivalent at Schwab. About 0.08%.

The "Index" version of these funds is important — Fidelity and Schwab also offer actively-managed target-date funds with much higher expense ratios. Always look for "Index" in the name.

Target-date funds have one big advantage and one small downside:

Advantage: zero ongoing decisions. The fund does the allocation, the rebalancing, and the age-based shift for you. Statistically, the people who buy a target-date fund and ignore it for 30 years often outperform the people who try to manage their own allocation actively — because they don't panic-sell during crashes.

Downside: slightly higher expense ratio than building it yourself (0.08% vs ~0.03%). Over 30 years on a large balance, that's a real but modest cost — maybe 1-2% of your final balance. Worth it for most people in exchange for the simplicity.

The slightly more engaged version: build it yourself

If you'd rather have direct control over the allocation, the DIY version is two or three funds, total:

The two-fund portfolio (most people, most situations):

• A U.S. total-market or S&P 500 fund (VTI, VOO, FXAIX — covered in Part 2)
• A total bond market fund (BND, AGG, or FXNAX)

Hold them in the ratio your age-based rule suggests (say, 80/20 at age 30) and rebalance once a year — when one side has drifted significantly from target, sell some of the overweight side and buy more of the underweight side, or just direct new contributions toward whichever side is now underweight.

The three-fund portfolio (adds international):

• 50-60% U.S. stocks (VTI or VOO)
• 20-30% international stocks (VXUS — Vanguard Total International, ~0.05% expense)
• 20-30% bonds (BND or equivalent)

The international debate is real — over the last 15 years, U.S. stocks have crushed international, but the prior 15 years was the opposite. The Bogleheads-style answer is that holding both reduces risk and you don't need to know which will outperform in advance. The "just buy U.S." crowd argues that the largest U.S. companies do plenty of international business through their multinational operations. Both positions are defensible; you won't end up dramatically richer or poorer either way over 30 years.

Either approach works. Pick the one that fits your willingness to keep up with the rebalancing. If you'd rebalance maybe once every five years instead of yearly, just buy the target-date fund — it does the rebalancing for you.

Common mistakes to skip

A few patterns to avoid:

• Too conservative when young. A 25-year-old in a 50/50 portfolio is leaving meaningful long-term returns on the table — for protection they don't need against volatility that won't affect them for 30+ years.
• Too aggressive near retirement. A 65-year-old in 100% stocks is fully exposed to the sequence-of-returns risk that bonds exist to cushion. If a 40% drop happens in your first year of retirement, you can be in real trouble.
• Changing allocation based on the news. Whatever you set up should be designed to survive a 30-40% market drop without you flinching. If you can't promise that to yourself in advance, your allocation is too aggressive. The single most expensive mistake in investing is panic-selling during a crash.
• Picking by gut feel. "I'll just go 70/30" is fine, but tying it to your age via a rule of thumb gives you a defensible answer when the market scares you. "I'm 30 and 110-minus-my-age says 80% stocks" is a much sturdier mental anchor than "I felt like 70/30 last year."

Bottom line: the whole Investing 101 series in one paragraph

You started this series with a question most people never get a satisfying answer to: how do I actually invest? Four posts later, here's the entire framework:

• Part 1: Why investing isn't optional — compounding makes waiting the most expensive sentence in personal finance.
• Part 2: What to buy — low-cost index funds, picked from 2-3 specific tickers at any major brokerage.
• Part 3: Where to put it — 401(k) match first, then HSA, then Roth IRA, then brokerage.
• Part 4 (this one): How to mix it — 110 minus your age in stocks, the rest in bonds, or just buy a target-date index fund and skip the math entirely.

That's the whole stock-market investing decision for the vast majority of people. Open the account. Pick the fund (or the target-date fund). Set the auto-transfer. Pick the allocation, write it down, and don't change it based on the news. Let compounding do the work for the next 20-30 years.

The investing part is now solved. The harder part — actually doing it consistently for decades — is on you. But the cost of not doing it (Part 1's math, again) is much higher than the cost of doing it.

If this series helped, share it with one person who's still trying to figure investing out. Four posts they can read in twenty minutes is shorter than most YouTube videos that say less.

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Investing 101 series:

• Part 1: Why investing isn't optional — the math
• Part 2: What an index fund actually is (no jargon)
• Part 3: Which account to open first — 401(k), Roth, HSA
• Part 4: Asset allocation by age — the simple version (you're here)

This post is informational and not investment advice. Historical returns are not indicative of future results. The allocation rules of thumb here are starting points, not personalized financial advice; consider talking to a fee-only fiduciary advisor for guidance specific to your situation.