Disclaimer: This is not a rant about sex in the movies or the "general fall of moral standards in the modern youth", but genuine question
In the book, all kids from the "Loosers Club" are 11. Beverly is described as a pretty girl, that barely started hitting puberty:
She paused (as she now almost always did) to look at her chest in the mirror, trying to decide if her breasts had gotten any bigger in the night. She had started getting them late last year. There had been some faint pain at first, but that wa s gone now. They were extremely small — not much more than spring apples, really — but they were there. It was true; childhood would end; she would be a woman.
And indeed, in the 1990 movie, the Emily Perkins that played her born at 1977 was around 12. We can definitely see that Beverly is a girl:
The gang (with exception of Ben who is madly in love with her) treat Beverly as "one of the guys".
In the 2017 remake, Beverly is no longer a girl - she is a young woman and so was Sophia Lillis born in 2002 (so around 14-15 when the movie was made)
Now, you could argue that 1989 girls try acting adult earlier than girls in 1956 (the original movie took place in 1956, the remake changes it to 1989), but this would be more characteristic for "girlish" girls rather for tomboy such as Beverly - so it seems that indeed this was a purposeful change made by director.
Has Andy Muschietti commented anywhere why he made such big change to this character?
Edit: I know about the (in)famous sex scene in the sewers where Beverly has sex with all of the boys from the Loosers gang, but it was not supposed to be erotic (most of the boys are too young to archive orgasm) but symbolic - as a sign of a bond and transition into adulthood. Still, that doesn't answer the question, why the "new" Beverly is so sexualised in comparison to both book and the 1990 movie.