You must begin to withdraw money from your traditional IRA at age 70 ½. There is a tax worksheet you must fill out to find the minimum required distribution, but basically, you take the total of all your IRA balances, and divide by your life expectancy. This is your minimum annual withdrawal.
In the case of a Roth IRA, there is no requirement to withdraw money. The entire account can be left intact for your lifetime and then passed down to your heirs who must then begin to take withdrawals.
For further information, visit this website: http://www.irs.gov/retirement/article/0,,id=96989,00.html