Extract birthday from profile for php evaluaton

Hi, this is probably an easy question for anyone with php experience, but I have none.

I'm using the rules module, and want to create an action based on the age of the user (i.e. under 18 or over 18). How to I extract the user's age from the birthday module (needed for the rules php evaluation)? This is what I have so far (extract from the if statement):

if ($user['values'][birthdays-age'] < 18)

I'd probably be embarrased by the code if I had php experience and saw what I've actually coded :-)