re: What happens to Josh from "Big" in the future?

He joins the army goes to Afganistan and goes for a hike.

Then the hilarity begins

I think he would be institutionalized by 18. There is no way anyone would believe his story and there is no way he is moving past it either. That "big" phase more then likely ruined his life.

If he cuts ties with Billy, he'll be fine. Susan, however, ends up in the crazy house.

An actual adult, with the mind of a 12 year old is functionally retarded in the eyes of the world. Also, if he stays big, he doesn't go to high school. He's essentially a middle-school dropout.
Also, his job success was contingent on his ability to relate to children's interests as an "adult". As his mind ages and develops, he won't be as in-tune with kids and would have to rely more on things like market research and the things normal product developers do. What made him special will be gone and he will no longer be an asset. Especially after his biggest supporter dies or retires, which probably isn't long into Josh's career.

Also, it's always struck me as odd that there was an implied sex scene in that movie. The woman was an adult and he was a child - in an adults body. I get why it's in the movie, but it always struck me as odd because when it's all said and done, she banged a 12 year old -- and we're supposed to feel good about that. Strange.