Occasionally you'll have to change the Version/Product numbers to make the outfit show properly, Hat has an overview here: ) For SimPE, change the 'age' category to 48.
(YA and adult can use the same files ingame as long as they're set as such, so usually you don't need special files for YA - you can edit the files in the Wardrobe Wrangler or SimPE. Can't remember if the tops are for all categories - not sure if I fixed these, but the male ones can also be useful combined with bottoms for summer wear). (And yes, it's very useful for checking skins - you can't use them when making the skins, but they're good to have afterward to check for issues. There's both top/bottom and a whole outfit for each age (toddlers only get whole), so you can pick which ones you want.
This set by Drachis (which I've had in my game since forever) requires no meshes (based on ingame meshes), except for the toddlers.
