Age verification

This isn't built in functionality but you could certainly write your own custom validation to do this.

You would write custom PHP that uses the gform_validation to validate the date entered in the date field and throw an error if they aren't old enough.

Here is documentation on the gform_validation hook:

http://www.gravityhelp.com/documentation/page/Gform_validation

Here is a complete tutorial example of how to write custom validation using this hook:

http://www.gravityhelp.com/documentation/page/Using_the_Gravity_Forms_%22gform_valiation%22_Hook