Are you sure he was a college boy, of did he just look young? (Maybe he was from corporate?) Anyway, I'm with BTD- trust your gut. Maybe a warning sign went off for a good reason, like perhaps it was a new card and he was being irresponsible with mommy and daddy's money, or it was stolen, etc. Could be he was legit, but something tripped your sensors...... Then again, you could just be a cranky old person.
(Kidding! I would have been a bit wary myself, but I really am a cranky old person.)