They couldn't find a teenager to play Paul? Again?
The book makes it clear that Paul is a 14 year old boy who is small for his age. Both prior films/TV shows screwed up when they used adults to play a young boy. Kyle MacLachlan and Alec Newman were poorly cast as Paul.
In the book after Paul and Jessica are given sanctuary with the Fremen, the story jumps ahead several years. Paul is nearly an adult and has a child with Chani by this time. This is a perfect time to switch over to a more mature actor.