It is not possible to change your age on TikTok. Once you have entered your age when creating your account, it cannot be changed. This is because TikTok uses your age to determine what content is appropriate for you to view. For example, if you are under 13, you will not be able to view content that is rated for mature audiences.
If you have entered the wrong age when creating your account, you can delete your account and create a new one with the correct age. However, you will lose all of your followers and videos if you do this.