Good coaches haven't spent years learning ancient secrets. Rees can watch film from other teams, go to coaches clinics, etc. to stay current with college offenses (and defenses). He doesn't need some 20 year apprenticeship to be a good coach.
Good coaches know how to relate to players in a way to get them to perform at their highest level. Rees is a gamble because he hasn't proven he can do it in an OC role, but the age/inexperience thing is overblown IMO.