Although you have the option to add a birth date to your profile, TikTok will not allow you to make edits to your birth date after it has been set.
It can be updated by going to your Profile, tapping the three line menu, and then Settings and privacy > Report a problem.
After this, you can choose Account and profile and editing profile and finally you can select ‘Other'.
Click the 'Need more help?' button and write some words describing the difficulty that your birthday is wrong.
To verify your true age, you'll have to submit a photo of a valid government ID or passport for support to update it.