According to wikipedia, the median household income in 2006 was just over $48,000 a year. So, let's just say this is your average lifetime salary and you saved 10% before tax (with 401k, IRA, savings account, whatever) every year. Finally let's also say you were getting 6% interest compounded monthly, and you did this from the time you are 40 to 65 (or 25 years).
$48,000/year -- putting $4,800 away a year -- that's only $400 a month
After 25 years, just from 10%, you will have $269,657 in your account!!
BUT if you started early, say at 25, and put away $4,800 a month till you are 45 and then added NO MORE MONEY when you turn 65 you will have over $595,000!
Talk about the power of compounding interest!