Building an Investment Portfolio by Age

You know your investor temperament, and you know the retirement rules. This lesson assembles the actual portfolio — what to own, in what proportions, at every age — and the small amount of maintenance that keeps it working for forty years.

If you have not yet read How to Start Investing and Investing for Retirement, start there. Everything below assumes you understand your reaction to market swings and the case for low-cost, broad-market funds.

The Core Principle: Own the Whole Market

A portfolio is not a collection of good stock picks. For nearly every family, it is a small number of broad-market, low-fee index funds held for decades. The case was made in Investing for Retirement and it bears repeating in one sentence: a fund tracking the S&P 500 at a 0.03% expense ratio (such as VOO) and one tracking it at 0.0945% (SPY) will perform essentially the same — before the higher fee quietly takes three times as much of your money. Fidelity’s FNILX does the same job at 0.00%, with the modest trade-off that mutual fund shares settle at the end of the trading day rather than trading live. For a long-term portfolio, that intraday liquidity is not worth paying for — and it is hard to argue with an expense ratio of zero.

Buffett’s instruction for his own wife’s trust — 90% in a very low-cost S&P 500 index fund, 10% in short-term government bonds — is the strongest endorsement the average investor will ever get that the boring portfolio is the right portfolio. The rest of this lesson is mostly about choosing your own version of those two numbers.

Asset Allocation by Age

Allocation — the split between stocks and bonds — matters more than any individual fund choice. Stocks provide the growth; bonds dampen the swings that make people abandon their plan at the worst possible moment. The younger you are, the more swing you can afford to absorb, because your time horizon does the repair work for you.

A reasonable starting rule of thumb: hold roughly 110 minus your age in stocks. Graham’s guardrails from The Intelligent Investor still apply at the extremes — the defensive investor should rarely go below 25% or above 75–90% in stocks once retirement is in sight.

These are starting points, not commandments. The honest adjustment is behavioral: recall the test from How to Start Investing — the day your $100,000 balance shows a $1,000 loss. If that scenario would genuinely cost you sleep, shift one row down the table regardless of your age. The best allocation is the most aggressive one you will actually hold through a bad year. My wife and I run different tolerances in our own accounts for exactly this reason — the plan you can live with beats the plan that is theoretically optimal.

The Three-Fund Portfolio

You can build the entire allocation above with three funds — and many families never need more:

1. A U.S. total market or S&P 500 index fund — the growth engine. VOO, FNILX, or your provider’s equivalent at the lowest expense ratio available.
2. An international stock index fund — optional but reasonable at 10–30% of the stock side, for the years when markets outside the U.S. lead.
3. A short-term treasury or total bond fund — the shock absorber. Low-cost options include VGSH, SCHO, SPTS, or FUMBX, all near 0.03%, all easier than maintaining your own bond ladder.

That is the whole machine. Every additional fund you add from here should have to argue its way in — most overlap with what you already own and add fees without adding diversification.

Core and Explore: The Speculation Rule

Graham’s rule from Security Analysis and The Intelligent Investor is house policy in our family: never mix your investing and speculating — accounts or mindsets. But the realistic version of that rule for most people, and especially for teenagers learning alongside you, is not “never speculate.” It is: cap it.

We use a core-and-explore split. At least 75% of every dollar invested buys the boring core — the index funds above. Up to 25% is the explore budget: the individual stock you believe in, the sector fund, the position you want to learn from. The split does the disciplining for you. When the explore pick wins, the position was small enough to celebrate safely; when it loses, the core never noticed. And both outcomes teach — by observation, with real money, at survivable scale.

The best way to hold yourself to this is to keep the two in separate accounts — separate brokerage accounts, or even a separate IRA. Keeping them physically apart means that when the explore side is losing, the core is not sitting right there offering itself as a source of funds.

You are only required to be right about the core. The explore budget exists to keep the gambling urge from ever negotiating with the retirement money.

Rebalancing: Once a Year, On Purpose

Left alone, a portfolio drifts. A strong stock year turns your 75/25 into 87/13; a bad one turns it into 68/32. Rebalancing — selling what grew past its target and buying what fell below it — restores the allocation you chose on purpose. It is also the only mechanism in the whole system that forces you to sell high and buy low automatically, with no forecasting required.

• Frequency: once a year is enough. Pick a date you will remember — a birthday, the new year — and put it on the calendar. Checking more often invites tinkering.
• Threshold: if you prefer rules to dates, rebalance whenever an asset class drifts more than 5 percentage points from target.
• In tax-advantaged accounts (401(k), IRA, HSA), sell and buy freely — there are no tax consequences inside the wrapper.
• In taxable accounts, rebalance with new money instead: direct fresh contributions to whatever is under target, and let dividends accumulate to the lagging side rather than auto-reinvesting. You get the same correction without realizing capital gains.

Staying on Track Through Every Cycle

Every portfolio you build will live through multiple drops of 20% or more — that is not a risk, it is a schedule. The portfolio above survives them mechanically. Whether you survive them is decided by the habits from How to Start Investing: automate the contributions so dollar-cost averaging buys more shares precisely when prices are down, ignore the commentary industry whose job is narrating every wiggle, and check the account on your schedule — once or twice a year — not the market’s.

A down year with steady contributions is not a setback; it is the years your future shares were on sale. The investor who keeps buying through the 2008s and 2020s of their lifetime ends up owning dramatically more shares than the one who paused to wait for clarity. Clarity is never announced. The schedule is the strategy.

Final Thought

Build it boring: a low-cost core sized to your age and temperament, an explore budget capped where it can teach but not hurt, one rebalancing date a year, and contributions that never stop. The whole design fits on an index card — which is exactly why it works. The next lesson, Tax-Advantaged Accounts, covers where to hold all of this so the tax code compounds for you instead of against you.