ORIGINAL: GMMAT
Nobody's saying that a 3.5 yr old is at his "PEAK". We're (the majority) saying he's beginning to be "mature" at that age.
I have no idea of when he reaches "Peak" "maturity".....but I (and the majority) feel he's at least beginning to be worthy of the "mature" monniker at 3.5yrs.
5.5-6.5 is the consesus IMO