If you have entered your age wrongly while registering for ur account in Google and now want to change your age, you cannot do it by editing your personal info or settings. You need to send a scanned proof of any authorized document like driving license.
So, to change your age, go to this link Apply for age changes to Google
, type in your correct name with which you registered your account, and your Gmail id. Upload a scanned copy of proof that proves your age and Submit the form.
Google will change your age as soon as it verifies your document.
Hope that helps!