That's kind of like saying that the majority of people who get cancer are old people, so we can reduce cancer rates by dying young.

The issue isn't strictly overpopulation. Its overpopulation combined with the amount of consumption. You can solve this either by reducing the population, or reducing consumption, or both. For a lot of people, there are compelling ethical, economical, and personal reasons for wanting to have children or at least avoiding restricting other people's desire for children. So in that case it makes more sense to reduce consumption.