No one know how old she was when she drank the elixir, ZUN art makes nearly everyone look like teenagers, and they're no official height chart for all the character. For all we know Mokou could had a sudden age increase when she drank the elixir.
You wanted to say that ZUN's art makes everyone look like bizarre blobs that somehow resemble humans. His (old) art is pretty damn bad and I'll eat my cat if you can tell the difference in age between those that aren't clearly little kids. Take Eirin for example, she's usually pictured as a tall and mature woman yet her in-game pictures make her look basically the same as everyone else. Now that i think of it, ZUN's art resembles yukkuri if you take away the bodies (lol).
