Assuming cell A1 contains their birthdate then try:
Code:
=TRUNC((NOW()-A1)/365.25,0)
now() will return the serial number for the present time.
you subtract their birthday from "now"
that will give you a number
divide that number by 365.25 (since every 4th year is a leapyear)
use TRUNC to just return the year (since people don't round up their age - after 21 
)
that should do it.
Regards,
Andrew