For being a police offer, the limit is definitely going to be the local jurisdiction, and is going to be dictated by how crime the location has; in general, the more crime, the more likely you are looking at a younger limit (such as 30-35) as faster reflexes is more necessary than wisdom (it's a life or death thing). However, in more peaceful situations, the age limit is likely to be older or non-existent, as the ability to make interesting decisions is more important, as the cop is more likely to act as arbiter than arrester.
If it helps...
FR