Testingcheatsenabled true, and shift-click on the gnome you want to change age on. If it has an age, you'll get the option to change it. My guess is you need two aging gnomes of any type to get babies, but the babies will all be and grow into 'normal' Mysterious Mr. gnomes.
All of mine are magical gnomes, but the post I was answering was asking about Sims 4, which I don't have. I've seen pictures of a gnome in it, but I don't know if
he's magical.
Baby gnome going for a 'swim':

Child near the Snowgnomes and elder sleeping on his cane:

I've got a teen gnome pic somewhere, but I can't find it right now.