Compound Interest: Grow the Tree, Eat the Fruit

This one is for my kids, and I want them to read it young — because the single most valuable thing I can hand them 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 up to. 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 — and earn about 10% a year. That’s 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 the same length of time, but a bigger leap, 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. Every time you sell to buy something shiny, 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. And the pile still grows. Why? Because by then the tree throws off more fruit each year than the kid eats. Take out $100,000 from something earning 10% on $21 million, and it 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. Even one tiny deposit, left alone for 89 years, becomes millions. Starting early is doing most of the work — not the amount.
• Kid B proves the watering. Adding savings on top of that early start is what turns “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. So 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 along the way — years where the tree looked half-dead. The whole plan only works if you don’t sell during those years. The math is easy; sitting still while it drops is the hard part.
• 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 — which, by the way, is why a “9-year double” feeling is honest too. The tree grows in today’s dollars more slowly than the chart’s number suggests.
• 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. Plenty of people can’t hold that line. But here’s the comfort: they don’t have to. Kid A did almost nothing and still ended up with millions.

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. $100 matters more at your age than $10,000 will at mine, because yours has more doublings left in it. Then water it when you can, and — above everything — leave it in the ground.

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

• #money
• #fire
• #compounding
• #habits
• #life