I think it depends on whether or not the outfit is an original EA/Maxis item or not. I don't know for certain about those game outfits as I've never tried changing them but I would think that you would have to clone them first before making any changes to them. Otherwise, you risk changing game files which might corrupt your game. But that's just my guess.
For custom clothing though, yes, you can change the category on any of those without issues and without needing to clone them first. I've done that sort of thing frequently as I use custom elder meshes which match the meshes for adult/young adult. And x'58' does indeed change the clothing to be wearable by those three ages. The hex values for ages are as follows:
Age Value in Property Set
0x01 - Toddler
0x02 - Child
0x04 - Teen
0x08 - Adult
0x10 - Elder
0x20 - Baby
0x40 - Young Adult
Changing the age setting on clothing means that the clothing will be using the same mesh for all the ages you select. It will not, for example, make the clothing "fit" an elder mesh which is different from the Adult/YA mesh. The elder version will take on the same shape as the Adult/YA while wearing that outfit.
This is directed at all those who are smarter than I (and the list seams to grow with each passing day). I have read several tutorials on changing the age value for clothing outfits through the use of Simpe and have some unanswered questions that I need help with. First; It is my understanding that prior to changing the file you must first clone the outfit through bodyshop. Why can't the value be changed of the original file? Second; It is my understanding that if you change the age value to 0x00000058 it enables the outfit for all 3 (y/a, af, ef) or am I mistaken?