Validate Multiple Inputs Together?

[eluser]dustinsenos[/eluser]
Heyo,

I'm trying to validate a user's age. In order to do so, I need to validate the combination of three input boxes:

month, day, year.

Am I able to do so using custom _callbacks? Can I attach a _callback on the *entire* form before it's submitted? Can I throw a 'form validation error' myself outside of the _callback hook?

Really appreciate the help,
D