With "young Kovu" revealed, I'm betting there's at most 4 "youngs" (the only major characters that appear as kids, Simba/Nala and Kovu/Kiara--I doubt it would go into the TLK2 side characters like "young vitani", that'd be a bit overkill). My role is also really minor, so these "duplicates" are probably not taking up a lot of roster space.
I can see the "young" versions as having more minor abilities, but I did notice that Lyra claimed just "Kiara" proper. That would be an easy role to claim (A major character, and prior to this lynch I definitely didn't come in expecting there to be young/old versions)--It makes the vanilla thing seem even more odd since if anything I'd expect an adult char to be more likely to have an ability than a "young" version...But I digress, saving this subject in the back of my mind for now