1. A teen skin is a combo of aesthetic and age, imo, but both can overpower each other. Give me a trendily dressed 40-year-old guy, and the teen aesthetic makes it a teen skin in spirit. Make him 60, and it'll still be a teen skin to me unless he actually looks 60. You can dress someone in funeral attire, but if the skin looks young enough, it could be a teen skin. There are cases where the character's face could be a teen's or not a teen's, and if the outfit isn't a e s t h e t i c enough, I won't count it as a teen skin unless I know the character is actually a teenager.
2. So long as it looks somewhat human from the torso up and has the right clothes, it can be a teen skin. Teen orc and centaur skins would seem teeny enough to fall under that category if given that trendy aesthetic.
3. I'd call it a teen skin because of how teeny the person in them looks. The face style just screams that to me. Teen skins can, imo, have varied clothing. Even if it's armor or a birthday suit, if the person looks teeny enough, I'd call it a teen skin.
4. That style is not necessary, but if you go with it, it's almost guaranteed to be a teen skin because of how young it makes characters look.
Overall, for a skin to be a teen skin, imo, it just needs to cross a certain line of teeniness, and that line itself is a little blurred.
Bonus:
1. The bias against teen skins is more of a bias against the generic trash that most teen skins are. I like to joke with a friend of mine that they're like a nation of inbred people since they look so disgustingly similar. That being said, though, the bias against them is uncommon, and I don't really know of any bias against non-teen skins. Both are in style and are gonna stay that way.
2. Based on what I've thought of and said in this reply, teen skins are a broader category than what I've initially considered, and making a skin that is a teenager is not necessarily uncreative. If you try to force the teen aesthetic in terms of clothes, though, you'll probably end up with something generic, and skins that try to look like everyday people or edgy gamers are generally a lot less creative.
3. Teen skinners are like other skinners. They can be highly terrible or really good at shading and stuff. Even if I hate a teen skin with all of my being, it can still have an undeniable amount of technique put into it. Teen skinners might be a little limited in their skillset, though, if they mainly do characters that look like everyday people.
I put too much thought into this lmao