She does, but only to the age of a college student, which is different from playing a child. And the thing about Merrily is that no matter who you cast, at some point in the show, they're going to be age inappropriate for the part and it's going to be a stretch of the imagination for the audience. That's just part of Merrily, and there's nothing wrong with it.
The other thing worth mentioning about the young parts Celia is playing, like Olive and Molly, is that these are parts in shows where the choice was explicitly made to have adults playing children. It's not like she's being swapped in for
real children in parts that are meant to be played by kids who are actually ten or thirteen or whatever. If they'd wanted to use real kids in these shows, surely they could have, but they've chosen to tell the stories differently.
A work of art is an invitation to love.