My roommate and I were discussing zero population growth and I pointed out that the median age would rise. This would likely increase medical costs and lower productivity. He argued that it would fall again later but that didn't seem to make sense to me. Then I realized that lifespan would also affect this. Constant or longer lifespans would also increase the median age while people dying younger would, of course, lower it.
I've been out of math too long. How the heck would I calculate that? I'm wanting to play with the numbers. Even pointers to a Web site would help.