How do you change your age on your google account?

Well, I was trying to change my birth year from 1997 to my real one 1987 for about 2 years so I can access 18+ contect.

Finally I found solution.

In google+ profile (about > basic info > edit) you can change your birth date to anything that will make your age under 18 (if your age on account is currently <18).

So you change it to tomorrow date minus 18 years.

It means you change it to date which makes you 18 tomorrow.

Then you have to wait 1 day and now you finally 18 on your account and you can change it to whatever you want.

Hope this helps.