Start Investing at 25 or 35? What Compound Interest Really Does
$300 a month at 7% grows to about $787,444 by 65 if you start at 25, and about $365,991 if you start at 35. The numbers, and how to start.
By the MoneyBackKit team · Updated October 27, 2026 · 5 min read
Compound interest means your savings earn returns, and then those returns earn returns too. The longer it runs, the faster it grows, which is why starting early matters more than almost anything else. Here's what $300 a month becomes depending on when you start.
$300 a month, starting at 25, 35 or 45
| Start age | Years to 65 | You put in | Growth | Total at 65 |
|---|---|---|---|---|
| 25 | 40 | $144,000 | $643,444 | $787,444 |
| 35 | 30 | $108,000 | $257,991 | $365,991 |
| 45 | 20 | $72,000 | $84,278 | $156,278 |
$300/month at 7% until 65
Starting at 25 instead of 35 adds only $36,000 of your own money, but ends with about $421,453 more. To catch up from 35, you'd need to save about $645 a month instead of $300.
Even ten years early is worth a lot
Save $300 a month from 25 to 35 only ($36,000 in total), then stop completely. Left to grow until 65, that becomes about $421,453, more than the $365,991 from saving every month from 35 to 65.
The rate matters too
| Return | Total at 65 from age 25 |
|---|---|
| 4% | $354,588 |
| 6% | $597,447 |
| 8% | $1,047,302 |
Over 40 years, a couple of percentage points changes the result a lot. That's why fees matter: a fund charging 1% a year more costs you a big share of the final balance.
How to start
- If your employer offers a 401(k) match, contribute at least enough to get all of it.
- Open an IRA if you don't have a workplace plan, or to save more.
- Low-cost, broad index funds keep fees down. Set up an automatic monthly contribution so you don't have to think about it.
- Keep money you'll need within a few years (an emergency fund, a home down payment) in savings, not in stocks.