There's this one University Sports Outfit I like and want to make it so its teen instead of young adult so I used bodyshop to export the file and copy then used SimPE to change the age integer string from 0x00000040 to 0x00000004 which should make it teen instead of young adult and since its copied the original should also stay intact, so I went back into bodyshop and imported it into the game and checked it out but it didn't work so I went back and checked the string and it was back at 0x00000040 even though I confirmed the change and saved in SimPE before importing so I searched up threads and found Wardrobe Wrangler which was much easier to do but it still did the same thing and went back to 0x00000040.

Answers to questions you might have:
I copied it but didn't rename the files at all
I saved it and went back to check if it saved right and it did before I started importing to game
I tried a total of 6 times

Teen bodies are shorter and a bit narrower than adult/YA bodies, so for teen outfits you need to shrink down and adjust the mesh and make a new set of recolors for it to work. It won't work to just set the age down.

It will show up, but sometimes with a broken/invisible thumbnail, and the outfit won't look good since it's using the wrong skeleon. Also, 0x40 is YA, so that only shows up in the Uni hood. 0x8 is adult.

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Without making a new set of meshes, you can gender convert for child female/male, and for toddler female/male, (plus baby female/male) - these don't use different bodies between the genders.

You can age convert between adult/YA/elder (but not gender-convert). Elder clothes usually have a different shape, so tops/bottoms may not look good.

This goes for hairs, clothes and most accessories (some hand-held accessories work for both genders for adults/teens)

Makeup can be age/gender-converted between all the ages.

To expand upon what Simmer2 said, teens and young adults don't use the same clothing meshes and the YA outfit is tied to the YA mesh.

If there is already a comparable teen mesh (without knowing the outfit you're wanting we can't tell) then it might be relatively easy to convert it to teen using BodyShop.

But if there isn't a teen mesh that's comparable, then you'll need to either find one or create a new mesh and put the YA texture files on that teen mesh.