This is because you and some others assume that some depth limit must be set do draw the line between OWD and AOW, but I think that such assumption is false and no such limit should be set at all. Therefore, the analogy with minimum age (like junior drivers licenses) does not work. People mature and develop with age, so some minimal age for driving must be set - and it is not surprising it is different between the states. However, what this has to do with depth? It's not like limiting age but like limiting speed for junior drivers. Most states limit juniors with curfew (i e, no driving from 12 am to 5 am in MA etc, and there is a reason for this) but I do not know any state that limits their speed to, say, 55 mph.