when we live till our 70's, 30 is considered young but nature has it otherwise, humans are at the pick of their physical strength at their early to mid 20's after that you start to slowly age, ofcourse you can work out and be more fit at your 30's than in your 20's but that's just a general rule. Anyway 30 it's just a number. I'll turn 30 in 2020 but for some reason i consider 27 to be the new stage of adulthood.