It depends on what kind of account you have.
If you have a traditional IRA, when you turn 70 ½ years old you must begin making a required minimum withdrawal from it each year. The amount of the distribution depends on how much you have saved in the account and your life expectancy, according to tables published by the IRS.
With a Roth IRA, you can leave the money in for as long as you want, letting it grow and grow as you get older and older.
The rules are similar for traditional 401(k)s and Roth 401(k)s. After you turn 70 ½, you must make required minimum withdrawals from a traditional 401(k). Not so with Roths. 
