Maria - broadly speaking, convert today's date into a unix timestamp, convert the birthdate into a unix timestamp and subtrct it from the first number. You'll end up with a new unix timestamp that represents the amount of time between the two dates (the person's age)... convert that to a human-readable date and you're set.
Try these URLs for more info:
php.net/strtotime
php.net/time
best,
Eric
http://ericmueller.org
