Is there any way to set an outfit for e.g a child sim for the time they'll turn into a teenager? I have the whole world filled with my custom sims, but I can't bring myself to let them age. I spent so much time on creating their outfits, so my heart breaks when they age up and go around wearing a silly hat, sports leggings and that blighted eye ring.

I don't think there's a mod for that.

I actually like to makeover sims, so I makeover all of my sims friends as they are kids, and then again when they age up to teenagers..
maybe you can take it as a challenge instead of an annoyance?

and if you can't be bothered going into CAS with every single one of them, you can always just randomize their outfits with MCCC so they're at least not as hideous..
(using MC Dresser > Randomize Part).

you can also grab Bienchen's Less Random Hats And Accessories that can help for the cause..
lessrandomhatsandaccessories here : https://deichschafblog.de/bienchen/...ds-outfit-mods/

These are very good tips, thank you!