[2019] Children not getting a profession when becoming Adult

There's a bug in the current production version where you can only assign from the settlers screen (L) but not the entity screen (E when a settler is selected). Or maybe the other way around. One of the two lets you. :S Or you can update to the beta build which has a fix in.

Keep in mind that children cannot have their profession assigned until they become adults.

As for getting into the beta, make sure you are in online mode of Steam. It won't let you enter the beta unless you are.