In the novel, Tim is the older sibling, computer wiz, and dinosaur enthusiast. In the movie, their ages and personalities are swapped (mostly) so that Lex is the older sibling and the computer wiz (while Tim remains the one who knows everything about dinosaurs).
According to IMDb,
After Joseph Mazzello was turned down for a role in Steven Spielberg's Hook for being too young, Spielberg told Mazzello that he was still impressed with his audition and would try to cast him in a future project. Mazzello was then cast as Tim in this movie. His casting led Spielberg to reverse the ages of the children.
Is that the only reason? If so, it seems weird that Spielberg would turn Joseph Mazzello down "for being too young" for one film, but didn't care that he was too young, and actually changed the ages of the characters to cast him for another film.