Inserting an age limit to my elgg site. How?

Hi, I wanna know how can I add an minimum and maximum age to my elgg site. I need an age limit for my site because when the site is done, the site is only for teenagers aged 13-19, which means that the minimum age is 13 and maximum 19. I'm not an age racist, but I'm just trying to make a safe social network for teens where the child abusers are not allowed. I hope you answer asap.

Bye.

P.S. And I want the age limit field to be in the registration page. And I don't want just a simple coppa, I want a birthdatepicker for the age limit :)