Chatham House has an article on Demographics which set me thinking. How important is the ageing population for its effect on the environment (e.g. global warming)? Old people consume but don't produce. However governments see full employment as a key goal. When an old person dies that reduces consumption. But the matching fall in production doesn't happen. Either automatic processes or government action restores production (making everyone else richer).
Of course this will be irrelevant in many possible future circumstances where production is limited by energy not labour shortages. In that case population at any age is irrelevant to pollution.