I was at work so I couldn't get any photo examples, but I made a few for you to hopefully help you visualize.
So here's my Young Adult version of Jannette, aged down to a teenager.
https://i.gyazo.com/7531956195da98e1aaf74a4d436f7b09.png
https://i.gyazo.com/b6446b95d633e8accb1c8f57f62d3c09.png
It's pretty easy for females, baggier clothing and messier hair. The young adult hair in my opinion is pretty "shiny" and well kept, just go with messier for a teens. Smaller breasts and butt also help.
The males is where it gets tricky, so I aged up the male in my signature to see what he'd look like. This is something I've actually never done before since in his story he can't age so it was pretty fun.
Teenager to Young Adult
https://i.gyazo.com/e4b383ab6b7be0357892e573cbbe6761.png
https://i.gyazo.com/04443f674052b9cb0dbbf543ac9c1bf3.png
I kept his wavy/curly hair, but I'd probably try to shy away from that in a young adult. Overall the teenager version has much more narrow shoulders and has a tinier waste.
Here's a zoom in of teen/young adult.
https://i.gyazo.com/761af5e56f3f9129bb3a8133792d10bf.png
https://i.gyazo.com/bd9927788f66edd46552bd8a3d9c2d96.png
The teenage version has very slightly bigger eyes, with a higher upper eyelid raise if that makes sense? Basically in a lot of art styles younger people tend to have more open/bigger eyes, so mess around with that. Freckles are also a really great way to convey youth, so I removed the freckles when I aged him up to a young adult. But the biggest different you can tell is the jaw/facial features. Teenagers have a much more noticeable baby face, so by broadening his face I was able to make him look older.
I hope this helps, sorry for the picture spam.
