Classroom Assignment · Print or Assign
The Life Stage Experiment
Ordinary compound interest calculators assume you invest the same amount at the same return for forty years. Real life does not work that way: what you can invest at 22 and at 52 are different numbers, and the risk you can afford changes with your responsibilities and your time horizon. The Life Stage Calculator models five stages of a working life, and every stage is yours to change.
Laptop required · Works as a single-period lesson · Print this page for the write-in version
Before you touch anything: two predictions
1. The default plan invests a total of about $529,000 across 44 years. Write down your guess: what is it worth at the end? My guess: $
2. Which stage do you think matters most to the final number: the teen years ($25 a month) or the peak earning years ($1,200 a month)?
Run 1: The default path
Press nothing. Just read the results and record:
Total contributed: $
Final balance: $
Growth (balance minus contributed): $
How far off was your guess?
Run 2: The late start
Set the first two stages (Teen/Student and First Job) to $0 a month. Leave everything else alone.
New final balance: $
How much money did those two stages actually contribute in Run 1? (Hint: $25 a month for 4 years, then $200 a month for 5 years.) $
How much did skipping them cost the final balance? $
Run 3: The early bird
Reset, then double the Teen/Student stage to $50 a month and cut every later stage in half.
Total contributed: $
Final balance: $
Compare to Run 1: who invested more money, and who ended with more?
Run 4: Your plan, defended
This is the real assignment. Set every stage to numbers you actually believe about your own life, then defend each one. For every stage, justify two things:
The monthly amount: what will you likely earn at that age, what will life cost (rent, family, everything), and what does that leave to invest? The return rate: how much risk fits that stage of your life, given your responsibilities and how long the money has to recover from a bad year?
| Stage | $ / month | Return % | Why this amount (earnings, costs, life) | Why this risk (responsibilities, time horizon) |
|---|---|---|---|---|
| Teen/Student | ||||
| First Job | ||||
| Career Growth | ||||
| Peak Earning | ||||
| Pre-Retirement |
My total contributed: $ My final balance: $
Reflect
1. Which single change moved the final number the most? Why do you think that is?
2. The curve bends upward instead of climbing in a straight line. Explain why, in your own words.
3. Look back at your defended plan. Which of your assumptions is most likely to turn out differently, and what could you do about it?
The calculator's return rates are assumptions, not promises. All investments involve some form of risk, and past performance does not predict future returns.
Teacher notes
The big idea. Time in the market beats size of contribution, and a real plan changes shape with life. Students discover both by breaking the calculator, not by being told. Runs 1 through 3 build the intuition; Run 4 is the assessment: a defended, life-shaped plan.
Numbers for the board (calculator defaults): Run 1 turns $529,200 contributed into about $2,115,000; growth alone is about $1,586,000, so the money made roughly three times more than the person. Run 2 skips only $13,200 of early contributions but ends about $292,000 lower: every skipped dollar would have been worth about 22. Run 3 contributes about half of Run 1 ($266,400) and still ends above $1.1 million.
What to listen for in Run 4. The defense matters more than the numbers. Strong answers connect the monthly amount to a believable income and cost of living for that age, and connect the return rate to risk: a 22-year-old with decades of recovery time can reasonably hold riskier assets than a 58-year-old five years from needing the money. Push back gently on plans where risk rises with age, or where a teen invests more than a plausible paycheck allows.
Common misconceptions. "The big late contributions matter most" (Run 2 is the rebuttal: early dollars have the most time to compound). "Growth is linear" (the bending-curve reflection is the check; listen for "the growth earns growth"). "These numbers are guaranteed" (they are not, and the note on the student page says so; it is worth saying out loud).
Grading. Grade the thinking, never the plan. Complete recorded runs, a defense for every stage, and honest reflections is full credit; the actual dollar amounts in a student's plan are their own business.