1. Once logged into your account, click your image or avatar at the top right corner of the YouTube homepage.
2. Click “Manage your Google Account” on the dropdown menu that appears.
3. Click the blue text reading “Personal info” on the left-hand column of the next page.
4. Click the arrow beside your birthday.
5. Click the pencil icon to edit your birthday.
6. Enter your new birthdate, then hit “UPDATE.”
Now log out of YouTube, and when you sign back in, your new age will be there.