You do not enter it. Well....actually you do enter it when you put your date of birth in My Info. The software will use your DOB to calculate early withdrawal penalties if they apply.
If you received a regular retirement distribution after you reached age 59½, the Form 1099-R should include code 7 in box 7 indicating that the distribution is not subject to an early-distribution penalty (or code T or Q for a regular distribution from a Roth IRA). If the Form 1099-R mistakenly has code 1 (or code J for a Roth IRA distribution) for a distribution made after you reached age 59½, you can claim an Other reason exception under Other Tax Situations -> Additional Tax Payments -> Extra tax on early retirement withdrawals.
(When determining whether or not a distribution is an early distribution, TurboTax does not use the date of birth that you enter in the Personal Information section. TurboTax only uses the code in box 7 of the Form 1099-R to do this. However, if TurboTax sees that you reached age 59½ during the tax year, TurboTax will effectively ask you to confirm that whether the distribution was made before or after reaching age 59½.)