It's difficult to deal in absolutes when describing an optimal population. To me the starting point is a philosophical acceptance that old is (generally) bad and young is (generally) good. That said, can we construct a model that describes an optimal age and age range for the planet?

Working backwards begins the process of describing public policy initiatives (health, income, access, etc) to achieve that goal. More broadly, what is the role of the nation-state? or transnational corporation in achieving/thwarting/enforcing some set of universally(?) acccepted objectives. I see major dislocations, across the board, as we wrestle with these issues.

Chronological age is not necessarily equal to functional ability. There are teens, twenty and thirty somethings who cannot do the physical work and mental work of some in their 60s and 70s. Plus there is the cumulative experience which elders have which gives them advantages in functionality.