Why are Lex's and Tim's ages switched in the film adaptation of Jurassic Park?

Question

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.

Answer 1

Joseph was going for the role of Rufio. Dante eventually took the role around the age of 16. This will put Joseph at 7 or 8.

Based on the role Rufio has to play in Hook, leadership of the Lost Boys, it's a fair call. He just was too young. Dante played Rufio well especially in the fight scenes.

Seeing that the casting for Jurassic Park would have happened much later than Hook, I don't think there is much in the reverse he could have did. He basically told Joseph to wait for the next project he had lined up for him. That was Jurassic Park. Just sounds like a simple promise Spielberg wanted to keep because he liked the potential of Joseph.

The first time (Hook) he wasn't so sure about it. The second time he decided to give it a go. Seeing that Lex would have to be around 7 or 8, the switching also allowed for Spielberg to bring in the idea of the crush on Dr.Grant^1,2.

So yes it is the only reason (as well as the crush on Dr.Grant), no it doesn't seem weird.

_{1: The Making of Jurassic Park (Duncan and Shay)
2: http://en.wikipedia.org/wiki/Jurassic_Park_(film)#Production}