はり·なり1second ago @Flora Bay hey, ignorant fool, bones, diseases, brain development happen slowly and steadily over time, there is no clear cutoff point between the stages, HUMANS classified the stages to represent the best cutoff points, but its not something biology came out and said "uh yea, 18 is when you become an adult, uh, 3 is toddle,... lets make 65 old age for the lolz" Show less