Is it normal to be very childlike as a teen-adult?

A woman who is 19 years old wears childlike clothing, like Naartjie (look them up if you don't who they are or what their clothes look like). She probably wears child's panties too, because her socks and accessories are also childlike.

I'm surprised most of the books she reads aren't for kids though, but she does tap her book with her pencil eraser when she reads, which looks retarded.

Her gestures and the way she behaves is very quiet, but childlike as well. Instead of growing up, she regressed a ton (I went to school with her, when she dressed normal). There might be autism involved though, because she doesn't talk, like I said befor she's very quiet.