I would say three things culminated my realization that I had grown up:
Getting together with a few friends I once believed were the shiznit and discovering they were morons. Selfish, over-indulgent, narrow-minded, ethically challenged. They hadn't changed, which means I must have been exactly like they were at one time in order to think they were cool. Unfortunately, they all have children and are either lousy fathers, or decent fathers but bad examples of what men are or should be like in general (as spouses, partners, or members of the community).
Losing one's health and independence has a way of smacking one into the real world where life happens, having to accept and make the best of whatever hand is dealt them, letting go of some things they once envisioned for themselves and changing one's notions about what is [or is not] important.
My dad passing away after watching him decline in a way that made him want to go with what little dignity he had left, and not being able to do a damn thing about it.