Limiting access to message area based on age
 

I would like to limit the access to a message area in my forums based on a person's age, say, under 21.

I have birthdate required when signing up.

How would I go about doing this?

Sorry for re-posting if this has already been covered - I did a search for "Based on age" and "Based" came up and "on age" was too short a criteria for the system to search on.

Thanks!