I think what MintFinch meant was..in her game, everybody can get pregnant. Not implying it is vanilla behavior.
And it is the same in my game, because mods. No..not children, that's gross.

Without mods and cheats, YAs can woohoo but not become pregnant. They can get engaged but not married too.

I would guess that technically you can set any sim's data to think that they are pregnant? But you wouldn't get a belly unless you have meshes for it, and you might have some weird results with animations and glitching if you pick an age category which really isn't supposed to ever be pregnant 0_o But, I mean, you can make an adult sim act like a toddler if you mess around with their data enough, so I don't see why pregnancy wouldn't technically be possible for any age. Just nobody would really want it in their game for anything other than an "ewww that's weird" experiment factor.

Well, out of curiosity once, I set a child Sim to adult in SimPE so that they would retain the same model but be able to do adult stuff. Then I made the Sim pregnant. The body turned invisible. Oddly enough, the birth animation worked right - the kid ended up going into the parent's arms just fine.



EDIT: Here's a few more screenshots from the experiment: