Recently I saw a casting at actorsaccess.com for a blond St. Pauli Girl. Jobs like this involve knowing that you legally need to be 25 to pose for an ad campaign or work on a commercial for a beer or alcohol product. So even though it is only 21 to drink and 18 to serve beer and alcohol at the bar to be in the ad for the product involves being 25 years old.
Just because you are 21 doesn't mean you can model in an ad. It is the law to be 25 years old. I think the reason is also because a 21 year old female can look VERY young, and a girl who is 25 still looks young but has a more mature appeal perhaps.