@beardalaxy
Simple logic:
If your brain power at 27 is 100%, and you need all 100% to make decision, do you lose this ability at 30, when you're back at 95%?
Nah, I've noticed the same issue, and it is rooted in the modern concepualization of adulthood. In most of history and for me still intuitive concept is, that when you hit puberty, you gain right to accept more responsibilities. You start asserting yourself, and with that comes the adulthood.
But, these people are missing the "now, I own my life" switch, and keep waiting for it sometimes up to 35 years. And when they FINALLY hit that point, that is what they consider the adulthood age.