How to use BuddyPress profile additional fields?


I have some fields added to user profiles (for example – birthday, about me and etc.), but I wonder how to use them except of just showing this info on user’s profile page.

For example, maybe some blogs are for 18+ (horror or something) and is selected as non-public blogs, how system checks who is 18+ and who is 18-.

I know I have written this question in very grammatically collapsed way, but the main thing is understandable I think.

So how system decides which user is adult and which is not?