59 1/2可以开始拿，没有penalty。
72岁必须拿：
Once you turn 72, you must start taking annual Required Minimum Distributions (RMDs) from your Traditional IRA. Your first RMD must be taken by April 1 of the year following the year you reach age 72. Every year thereafter you must take an RMD by December 31. The amount of your RMD is calculated by dividing the value of your Traditional IRA by a life expectancy factor, as determined by the IRS. You can always withdraw more than the RMD, but remember that all distributions are taxed as income. If you don’t make withdrawals, you’ll have to pay a 50% penalty on the amount you should’ve withdrawn.
