This is a great plugin and I want to suggest a feature: a simple date field (without "calendar") and date checks (e.g.: given_date <=> mm/dd/yyyy, today - given_date > mm/dd/yyyy).
I need this feature to accept e-mails only from 18+ y.o. people
TIA and sorry for my english
PS: Can I do this check by using my-functions.php ?
No unfortunately, there is currently no _proper_ way that allows you to incoporate your own validation logic, outside of regular expressions and the other config settings.
What you could do is *if it's a must have*, is to hardcode it:
in lib_validate.php for non Ajax submissions
using jQuery on onclick handlers to prevent Ajax submissions