At what age can you start withdrawing from a 401K account?

59 and 1/2.

If you mean: at what age can you start withdrawing without incurring the 10% early withdrawal penalty, then the answer is either:

1. Any time in the calendar year when you turn 59-1/2, or
2. If you are age 55 or older in the year you are separated from service then you can start taking distributions without penalty.

The 59 and 1/2 age does mean you avoid the 10% early withdrawal penalty but you still pay ordinary income tax on the amount withdrawn. If possible, you can let the money grow until age 70 and 1/2 at which time partial withdrawal becomes mandatory.