I'd say life doesn't really begin until you're on your own, so around 18 for me, but I'd really say that life picked up around the late 20s, early 30s. So, at 37, if I look at everything that I've done in the last 10 years, and now knowing what I want to do, I've still got three lives to live.
Maturity, focus, concentration, self confidence all ebb and flow with time and aren't guaranteed to be monotonic. Civilization allows us to do great things, but it also enables the individual to be a weaker total person while being particularly skilled in one area.
Is that actually the case? I feel like a hunter/gatherer society isn't particularly well rounded - it's just a couple of skills that one lives or dies on. I wouldn't say that a caveman was well rounded, instead they were just ignorant of everything outside of "find something to eat and kill, then mate with something else".
Absolutely. Since I've turned 30 I've gotten married, found my career, and bought a house. I've really only figured out life in the last 5 years or so. I've got many lifetimes to live as well.