I have heard directly from people from NZ and Australia that one of the problems they have is that the Islander kids are enormous compared to the white kids at age groups. This puts off the white kids ( and maybe their parents) who are turning to football instead. They need to look at size as well as age I’m told, in order to keep them all playing.
I thought that NZ organised their youngsters by size/weight rather than age. It's often mentioned in pieces about how that compares to how things are set up here & why it's better as it means that smaller kids don't lose interest in the game as they're not being thrashed by much larger kids who are the same age as them...