Which populations have the most stable size and density?

Populations are a collection of several organisms belonging to a single species and living together in the same territory. The most stable number can boast of populations in which the age category is stable, that is, younger and stronger individuals easily crowd out the old and weak, and those, in turn, do not pretend to compete and die quietly. But, if we assume that such individuals will also reproduce, then this will lead to the fact that the number of young individuals in the flock will increase, and the competition will increase, which is unacceptable for a uniformly stable population.