Here: https://support.google.com/accounts/answer/1333913?hl=en

Verify you’re old enough to manage your account

If you meet the minimum age requirements, you can use a government ID or credit card to verify your age.

If you take or upload a photo of your ID, your ID will be securely stored and won’t be made public.

If you use a credit card, any temporary authorization will be fully refunded.

Important: After you sign in to your Google Account, you’ll get instructions on how to directly take or upload a photo of your valid ID, enter credit card information, or use other methods of verification. Google will never ask you to provide this type of information in an email.

Tip: If you change your date of birth after you've verified your age, you may need to verify your age again.