The primary reason populations are getting older is due to lower fertility rates. A fertility rate of "N" results in a generational ratio of 1 person of the cur

The primary reason populations are getting older is due to lower fertility rates. A fertility rate of "N" results in a generational ratio of 1 person of the current (older) generation to (N/2) of the next (younger) generation. So a fertility rate of 2 means the current (older) generation will be exactly the same size as the subsequent (younger) generation. But if we assume a fertility rate of 1, which is the direction most developed countries are headed, then each successive (younger) generation will be half as large as the (older) one before. This means society will be disproportionately composed of the ever-more elderly, with socioeconomic chaos, or Logan's Run, soon to follow. This would, counterintuitively, be true even if life expectancy was low. If we all died at 50 you'd just end up with a society with a larger and larger chunk of your population around 50 years old, until eventually there was no population left at all. And, vice versa, a fertility rate of e.g. 6 (as is the case in some places in Africa), means that each successive (younger) generation will be 3x as large as the previous (older) one. So you will always have extremely low mean and median ages in this context, again regardless of whether life expectancy was high or low.