How big is the gap between starting investing at 25 and starting at 35? Assume an annualized return rate of 7%, investing $20k each year until age 60:

Starting at 25: Investing for 35 years, total investment $700k, estimated final assets about $20k, with returns of about $700k.
Starting at 35: Investing for 25 years, total investment $500k, estimated final assets about $3.32M, with returns of about $714k.