As well your jaw never really stops growing, it just slows down a lot, as well as joint degeneration and remodeling results in the mandibular (lower) front teeth being forced against the upper front teeth, and usually the lowers buckling resulting in the lower crowding you see (especially in men as they tend to show lower teeth a lot more than women) in older males. I'm looking at you Will Ferrell. Retainers keep the teeth in the same relative position as the bone moves around them