Why did Menma appear as a grown up? In Ano Hana, Menma died when she was a kid, but then a few years later when she appeared again, her ghost was a teenager. What exactly happened here?
Jintan still has romantic feelings toward her, as do the other male
characters. There are also some scenes (at least one, and I think more)
where Menma is sitting on Jintan's crotch and moving around, which
naturally gives him nosebleeds and at least once causes him to pass out.
The implication is that she has no clue what kind of effect this has on
him because she died before learning about such things.

It seems likely that the creators did not want to show that kind of
scene with Menma looking like a child. Kodomo no Jikan ended up not
being adapted for the US market because of such content. The late-teen
male characters having romantic feelings toward such a young-looking
character would also tend to squick people out.