Assuming the same amount of time has passed in the Monsters world as it has for us, that means Boo (or whatever her real name is) would be in high school right now.
At the end of Monsters, Inc., Mike and Sully successfully returned Boo back to her bedroom, but her door was shredded so that there wouldn’t be any more escapes.
However, thanks to Mike’s collecting all the door pieces and putting them back together, Sully was able to revisit his young friend again.
It was a heartwarming moment, and no doubt there are many fans nowadays who are eager to know what happened to the girl.