Custom coppa age

[piotrc]

Hi,

I'd like to change Coppa default age from 13 to 15 years old. This is because in my country are different rules about children. I've done modifying language pack but i want also stop users below 15 years old at the level of registration.

What should I change in register.php code to do it?
My version is: 3.6.1

Peter