Does she read that much older then Billy?
Maybe just more mature, they could be twins.
Hopefully they can still be revealed as long ago separated twins.
As to the movie, Grace Fulton(Mary) is definitely older, but might be playing Mary as younger than the actress is.
Maybe they were anticipating a growth spurt from (Asher Angel)Billy , and that in sequels they would look closer to the same age.
And they both age up to about the same when powered.
Actors Zachary Levi(adult Billy) & Michelle Borth(adult Mary) are abt. 2 years apart.
Although right now they are definitely not reading much as twins. Which is a shame.
Of all the changes to the mythology, this change (and blond Freddy) are what irk me the most.
Yeah hate losing that too, although she may have been bumped from one family (the Bromwell's) to a home. So by contrast knows and appreciates what it means to lose a stable home.And she's in a foster home going by the name Bromwell, kind of defeating the contrast of her growing up with rich and loving foster/adoptive parents to contrast Billy's tough time.
While Billy never even experienced even that.