Age or weight?

I went to watch Grandsons 1 and 2 ( by age of course ) today in a Bristol  Rugby sevens competition. They play in under 15s and under 12s teams.
However the difference in the size of boys by weight is completely different and appears to lead to unfair competition and some unhappy faces on the smaller boys....in my opinion. I understand this is a controversial topic at the moment with different views in Northern and Southern Hemispheres.
So..enough said.
Grandson 1 in blip above and 2 in extra photos.