Depends on the genre we'll put her in :D
Magical girl: 12-16 (In some cases they go down to 10)
Comedy: 16 (Except in cases where her appearance are supposed to chock people when hearing her age, then she's closer to 50 and has a teenage daughter.)
Romance: 12-19 (Usually in the range of 15-19, except for in magical girl genres)
Horror: 15-16 or 18-19. Every single time!
Hentai: We'll just say she's legal and it will be fine. She's just a drawing so they can't prove anything :D
After having watched a shit ton of anime, I've realized that you really can't judge the drawing's age by its appearance, except in cases when it looks like this:
This is a clear cut case.
Otherwise there are too many cases where children looks like adults and adults like children, so it's mainly up to interpretation if you're going with anime logic, if the creator hasn't stated a specific age.
