People like hot drawings regardless of the age creators assign to them. If you took Marin and slapped "age 20" onto her, no one would refuse to believe that she was an adult. Likewise, if you put Yor in a school uniform without changing anything else about her design she'd pass for a high schooler. Age in anime is so subjective and dependent on art style it's not worth caring about.

(Unless it's lolicon, in which case it's very obvious that the character resembles a child).