How to change your age on google account
If you registered without your google account, you will not asked by google about your profile information. Google will ask create google account only.
so if you wanna use google product like adsense, google will reject with reason your age is not enough cause your age is same when you create google account.
Now you must change your age first before you apply google adsense, follow link to your Google account 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.