50 rimworld doesn't have children (yet). Also in rimworld xp goes up when people do a job that uses it. How much they like it changes how much (no interest has a 0.5 modifier, interest has an 1.0 modifier and burning 1.5). However if a skill is a certain level and it isn't used it slowly looses xp and sometimes even levels. I am not sure though what triggers the loosing of levels and when it starts to decay what I do know is that it even slowly decreases when someone is sleeping or even when someone is grabbing the stuff to do the job >_<. I really hope it is just a temporarly way to make sure you can't have all your colonists level everything to max.