When Jack first goes to school and sits down in his desk for the first time, you can see his book bag and a broken desk to his right. In the next shot, he reaches for the book bag and falls over. A second later, the desk is shown breaking as he sits in it - the same desk that could originally be seen the first time he sat down. The scenes appear to be reversed.
When Jack goes into his 5th grade classroom for the first time, he goes to sit down at an all-wood desk behind a boy with blond curly hair. After he sits in it, a front view shows him in a green metal and wood desk sitting behind a girl with black hair.
When Jack comes down the stairs in his house in a cardboard box, he comes down onto a hardwood floor at the bottom of the staircase. When his Mom sits down on the stairs, a runner (rug) is shown at the bottom of the staircase.
When Louie stops Jack after school, asking him for a favor, Jack is wearing a pink shirt under his jacket. When Louie takes him aside, just before he meets Louie's mom in the courtyard, his shirt has changed to red.
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.