is it possible easy to check if a person is age of 21?
like the way facebook does it. with 3 select boxes?
day month year?

Hello Denis Hoffman.

Yes it's possible using the rules.
The following two videos will answer to your question:

Validate for a Maximum and Minimum Number of Selected Items

And the following video will show you how to manipulate the extension when you got to use month and year fields for validation

Set a Validation Rule for Expiration Date