Trying to validate user's age through DoB

Trying to validate user's age through DoB

We have a form where a user enters their birthdate. We'd like a rule that counts whether the user is over or under 25 years of age. This amounts to 9125 days elapsed between today and an easy way to do this would be to use conditional rule Is Before [x] Day(s) and act on the output of that rule. Forms won't accept that many days. The rule takes 1000-or-so days, then breaks at validation and won't save a value like 9125. How else can we achieve this?
And while we're at it, why would the form limit days count like that? It's a pretty simple calculation for a computer ;)