Corrected entry: Toward the end of the film when Derek flashes back to Brennan helping him fly a kite, the two little boys are roughly the same age - about 6 or 7. They are 3 years apart, Brennan would be quite a bit taller at that age.
Bishop73
30th May 2019
Correction: The actor playing 9-year old Brennan (Bryce Hurless) was in fact 9 years old at the time (or close to turning 9 depending on when his scene was shot), while the actor playing 6-year old Derek was 7 at the time. While in real life, Hurless isn't very tall as an adult, children experience different growth rates, especially during puberty. So a short 9-year old that looks 6-7 to you could grow up to be 6'3", which I feel was part of the reason they cast the young actors as they did.
Bishop73