Compound Interest: Grow the Tree, Eat the Fruit

The most valuable thing I can hand my kids isn’t money. It’s time in the market. A dollar invested at 11 does work that a dollar invested at 41 can never catch. The earlier you start, the less you ever have to add.

So I built a chart. Two imaginary kids, both starting at age 11 with the same $3,000. I followed them to 100. The numbers below aren’t a promise — they’re a model. But the shape of the lesson is real, and it’s the most important money shape there is.

The one rule the whole chart runs on

Both kids put their money in a broad index fund — think the whole S&P 500, the productive engine of the economy I keep coming back to. They earn about 10% a year, roughly the long-run historical average.

Here’s the magic in one sentence: at 10%, money doubles about every 7 years.

That’s the Rule of 72 — divide 72 by your return, get your doubling time. 72 ÷ 10 ≈ 7. So $3,000 becomes $6,000, then $12,000, then $24,000. Each step takes the same length of time. Each leap is bigger, because you’re doubling a bigger number. Early on it crawls. Late on it explodes. Same engine the whole way — it just doesn’t look powerful until the numbers get big.

Kid A: plant once, never touch it

The first kid does exactly one thing. At 11, plants $3,000. Then never adds another cent, and — this is the hard part — never sells.

Read that last row again. One $3,000 seed, planted at 11 and left completely alone, becomes $14.5 million by 100. No saving. No skill. No second deposit. Just refusing to dig it up.

That’s the tree. You plant it once. The whole job after that is not chopping it down. Sell to buy something shiny, and you don’t spend the fruit — you burn the tree that makes the fruit. The miracle isn’t picking the right stock. It’s outliving the urge to touch it.

Kid B: keep watering, then live off the fruit

The second kid starts identically — $3,000 at 11 — but does two more things. First, keeps feeding the tree while young, with whatever they can:

• Ages 11–15: $500 a year (allowance, small jobs)
• Ages 16–18: $1,500 a year (a teenager’s earnings)
• Ages 19–24: $4,000 a year (first real paychecks)
• Ages 25–44: $15,000 a year (steady saving on a normal income)

Then, around 45, the tree is big enough to live on. So the kid flips from feeding to harvesting — pulling money out to live, and never working for pay again:

• Ages 45–64: takes out $60,000 a year
• Ages 65–79: takes out $80,000 a year
• Ages 80–100: takes out $100,000 a year

Watch what happens:

Look at the surprising part. From 45 on, this kid never earns another dollar and pulls out more each year — $60k, then $80k, then $100k. The pile still grows. Why? By then the tree throws off more fruit each year than the kid eats. Take $100,000 out of $21 million earning 10%, and the pile made $2.1 million that year. You ate the fruit. The tree got bigger anyway.

That’s financial freedom in one image: the day the fruit feeds you faster than you can eat it.

The two lessons, side by side

Put the kids next to each other and you get the whole philosophy:

• Kid A proves the seed. One tiny deposit, left alone for 89 years, becomes millions. Starting early does most of the work — not the amount.
• Kid B proves the watering. Savings on top of an early start turn “comfortable” into “free at 45.” Time builds the tree; savings decide how big.

Neither kid is smart. Neither picks winners. Neither times the market. They just (1) start absurdly early, (2) keep feeding it while they can, and (3) refuse to chop it down. That’s the entire game.

Where I have to be honest

I don’t want my kids to read this as a guarantee, because it isn’t. A model is a clean story. The real world is bumpy. Here’s what the tidy chart hides:

• 10% is the past, not a promise. It’s roughly the long-run average, but it came with brutal crashes — years the tree looked half-dead. The plan only works if you don’t sell during those years. The math is easy. Sitting still while it drops is the hard part. And past returns can’t prove future ones — that’s the whole point of the turkey, who had excellent data right up until Thanksgiving.
• Inflation eats some of it. That $14.5M at 100 won’t buy what $14.5M buys today. After inflation, the real doubling time is closer to 10 years than 7. The tree grows slower in today’s dollars than the chart’s number suggests. See what a dollar used to buy in the inflation calculator.
• The savings still take discipline. $15,000 a year through your late twenties and thirties is doable on a normal income — but only if you keep your spending below it, every year, the way I write about in living below your means.

So I’m not promising my kids a fortune. I’m handing them the one lever that’s almost entirely in their control: start now, and don’t dig it up.

The homework

If you’re young and reading this: plant the seed today, however small. Your $500 matters more at your age than $1,000 will at mine, because yours has more doublings left in it. Water it when you can. Above everything, leave it in the ground.

Then run your own version of this chart — your seed, your savings, your years — in the FIRE calculator. Seeing your own numbers bend upward beats reading about mine.

Grow the tree. Harvest the fruit. Never chop down the tree.

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