I think a lot of has it do with clothing/hair style and personality rather than actual looks. At least for me. Like, based on Eliza's personality, I feel she and Bob should definitely be adults and not young adults. Like the whole idea of her character doesn't seem like a young adult to me. She seems way too mature.
Catrina Lynx, on the other hand, I can defintely see being in her 20s or early 30s as well at most. Although, this could be because I'm only 23 myself, have never had a boyfriend yet and I'm a bit of a crazy-cat lady too. Lol. And same with Ulrike, I feel she's a young adult as well. Actually, I find Maaikee to be more "unrealistic" as far as being a young adult goes compared to Ulrike. And as I mentioned, I think it has to do with her hairstyle and the way she dresses. Same with Supriya Delgato. Looks-wise I can defintely envision her as a young adult, but her clothing style makes her look muuuuch older. And I can't remember who numbers 5-8 are, but yeah, they defintely seem like adults to me and not young adults.
And on the flipside, I think there are some premades who look too young to be adults and yet are classified as such, like Lana Mckinnon for instance.
https://static.wikia.nocookie.net/sims/images/4/44/Lana_McKinnon.png
I am constantly thinking she's a young adult based on how she looks/dresses but she's one of the professors in Discover University. Which is super off-putting for me. And although he looks like an adult, I feel Marcus Flex should be a young adult based on his personality. I mean, of course adults can still be non-committal, but the whole "unemployed, non-committal" thing fits a young adult better in my opinion. Plus, there's the fact that he lives with three young adults who are clearly supposed to be 20 somethings, although I do normally have to make a few changes to Jade to make her better represent her age. (ie I give her a different career and replace the "neat" trait with romantic normally.)
This makes me want to conduct an experiment with some of these characters, and see what a difference clothing + hair can make, lol.
Also, for reference, I feel the "Adult" stage starts somewhere between 35-40 on Sims.