Of course. But since he got them from a pet store, chances are they are already adults.

Saying that, don't be surprised if they don't give you many babies...or any for that matter. Mice at stores are usually older. The best way to start breeding groups is to get a pair of just weaned female mice, raise them up to about 10-12 weeks old, then put them and a young adult male in a totally new enclosure (no established smells or territory). Works great with good productivity.