On the scene where Jack's mother is talking to her husband on their bed she mentioned how kids will react with a 1'80 m classmate (talking about Jack) a in fact Robin williams height is 1'70 ; some parts of the movie they try to enfatize his height even do it's not very tall
The quote, "This place is for fun," by Al Nalbandian as the principal is a reprise of one of his few lines in American Graffiti, in which he is also associated with Francis Ford Coppola. In that film Nalbandian innocently affirms for the Pharoahs gang that the miniature golf spot was for fun, although the gang had, moments before, broken into the pinball machines much to Curt's horror.
In the last scene, after the graduation, there is a red sign in the background as Jack leaves in the car with his friends. The sign says "Seven Years Later", the same as the text when the movie jumps to the graduation.
Jack ages at four times the standard rate and has always been this way. This doesn't necessarily mean that all his metabolic functions have been so accelerated. For example, it's okay if his beard doesn't grow at four times the standard rate.