at 10 % compounded , which is approximately the average annual 10 year growth rate of the s & p 500 stock index , 28.654 dollars grows to 500.000 dollars over 30 years , a hefty sum around retirement at age 70 , starting to invest at age 40 , to pay for living expenses at age 70+ , when you wont be able to make as much money then , or even any money at all .