Seriously thanks for the criticism. IMO, the toon morph helps take them out of the uncanny valley by making them less real. Maybe like you said I need to push it further. What about the blonde one makes her look so old do you think? Is it her skin? She does have a lot of blemishes. Maybe I went too far with that. I'm aiming for early 20s.
I don't think it's any one particular thing, but multiple things all at once. I can start by mentioning that people can appear older (or younger) in a photo from a particular angle and not from others, so it's entirely possible she'll look more reasonable from other angles. Could even look better alone - relative to the other three, her head has a more adult/less toon proportion, and larger heads make people look younger, so by comparison alone she looks older there.
Another comparison thing that might not give the same effect were she alone - she's got a stockier build, not in a "I've got an ass" way, but in a "I've had three kids and I still look this good" way.
The shirt on her is hanging in such a way, relative to how form-fitting the clothing on the other three, that it comes across as her trying to hide a bit of a stomach pooch, but that may just be my own reading of it.
Given the number of freckles on her chest and the tone of her skin, it doesn't come across as freckles but as years of sun damage - something most 20 year olds just haven't had the time to develop.
And there's something about her face that I can't quite put my finger on that makes her look a decade older than the other three at least. It's like she doesn't have baby fat from still being young, but is developing middle-age slow-metabolism weight.
Any one of those on it's own wouldn't be an issue. All of them together comes across as someone much older.
And then, of course, the toon thing might also not help - not just proportions, but toons generally have smooth, even skin and if they have freckles, they're much darker than the skin - the exception being old characters. Dark, almost black dots on a toon's cheeks and chest? Freckles. Midtone spots? Liver spots.
I'd say start with the skin - either lighten it up so it's clearer that it's not sun damage, darken the freckles to make them more stylized, reduce the number of them, or a combination of all three.
