Usually all I change after they age up is their clothes, hairstyle and in the case of teens & older, make-up. I haven't had many children in my game yet, but with the ones I do have, they've been mostly genetically blessed. But, every once in awhile, I get a chin-less wonder and I can't just let that slide.
I try to be as subtle as I can with the plastic surgery on them though. For instance, Greta Gamble, the Gen 2 heir in my Random Legacy challenge I began a long time ago on my old laptop. Please excuse the horrible graphics. Anyway, she aged up to a teen looking like this:
https://i.imgur.com/N4BWPGG.png
And after a few tweaks:
https://i.imgur.com/7POFzEc.png
I gave her a chin, changed her nose slightly and moved it up a tad. That's about it besides clothes, hair and make-up. Honestly, she was kinda cute before, but I think she looks much better now. I dislike Sims without a chin. It's just my personal preference. *shrugs*