Same deposit, different start
Two savers put away the same $100 a month at the same hypothetical 7% a year until age 65. One starts at birth (someone else saves for the first years), the other at 18. The only difference is 18 years of deposits and growth at the front.
The numbers from the video
With interest compounded monthly at 7% ÷ 12 and deposits at the end of each month, the formula is FV = 100 × ((1 + 0.07/12)^n − 1) ÷ (0.07/12). From birth, n = 780 months and the balance at 65 is $1,583,822.61. From 18, n = 564 months and it's $438,642.92. The gap is $1,145,179.69, about 3.61 times as much, for $21,600 of extra deposits.
Why the early years matter so much
Money deposited in the first 18 years gets the most time to compound. At 7%, money doubles roughly every 10 years, so a dollar added at birth has about six and a half doublings to go by 65, while a dollar added at 18 has under five. Most of the final balance is growth, not deposits.
Monthly vs yearly-equivalent compounding
The video's figures use the monthly rate 7% ÷ 12. Some calculators instead use the monthly rate that compounds to exactly 7% a year, (1.07)^(1/12) − 1, which gives smaller totals: $1,419,716.96 from birth and $407,589.57 from 18. Switch the compounding option to compare.
What this page leaves out
The rate is hypothetical and constant; real returns jump around and can be negative. No fees, taxes or inflation: $1.58 million in 65 years will buy much less than it would today. The page doesn't model any particular account type.
Sources
Last reviewed: Oct 9, 2026