I can spawn a baby zombie fine using the command /summon Zombie ~ ~ ~ {IsBaby:1}. However if I swap Zombie for Skeleton: /summon Skeleton ~ ~ ~ {IsBaby:1}, an adult skeleton is created. If I understand it correctly the mechanics for baby mobs are identical between zombies and skeletons, so this should work?
I have uninstalled all mods and the problem persists.
There's no such thing as baby skeletons.