On first appearances, Jack is a fairly entertaining sentimental comedy. But the story reveals truths that many people don't realise, or even if they do, they hide it from themselves and others.
Robin Williams plays Jack Powell, a boy whose cells are dividing at a rate four times faster than normal. At the age of 10, Jack looks like a 40-year old. Shielded from his peers by his protective parents until that age, he finally enrolls in the fifth grade with a desire to explore life to its fullest. The movie focuses on how he copes with his unique condition as he goes through school.
If Welcome to the Dollhouse was too negative in its portrayal of how children treat other children, Jack is too positive. But the feel good nature of this movie is probably what attracts the audiences, and I suppose that's all that counts as far as Hollywood is concerned. While there are plenty of humourous moments, both the main comics in the movie, Robin Williams and Bill Cosby, seem highly restrained. But Williams is indeed the right choice for this role, since he plays a kid like a kid (not a kid who's grown up), and a shy one at that, and there are moments when he really shines. Comparisons to Big are inevitable, and personally, I preferred Williams' performance over Hanks'.
The movie only slightly touches upon the importance of being a child throughout your life (heck, the movie for the most part shies away from anything remotely serious). Adults create a existence based on their illusory self-worth, but in the grand scheme of things, their existence is meaningless, just like that of the butterfly that Jack encounters in the movie. But as long as you're here and alive, you might as well make the most of it. The people who make the most of life are little children, since they don't have the burden that adults bear. In a world where more and more freedoms are taken away from children, in the guise of protecting them, and in a world where children are regarded as second-class citizens, Jack serves as a reminder that they are far far better creatures than most adults are.