I do think that's going to be an issue even for the stock market. Seniors are going to be spending their savings. So, that money is going to come out of the investment markets of all kinds, and go into day-to-day consumption. Will the youngsters buy up all that stock? Honestly, I'm skeptical. You only have to look to Japan, which is some years ahead of the US in this aging curve to see what's happened. Both housing and stocks are well below their peaks, with no signs of recovering to those previous highs.
My feeling is that stocks will less affected than housing. Stocks are easy and cheap to buy and sell. So, there's no rush to liquidate, and savers can easily add to their positions. Housing is hard and expensive to sell. Now, I do think technology will help with the costs (sorry agents) just like it did for stocks and travel. But, it will remain less liquid than most other investments. That means it will be one of the first assets people unload when they're shifting to retirement. Locations with population loss may well end up with a significant surplus of housing, even if other locations continue to do OK. Detroit gets discussed a lot here, and its already in that situation. I doubt Detroit ever turns into a ghost town, but you only have to drive through one of the numerous ghost towns scattered all over the country to realize the value of a house can indeed fall to zero. It doesn't take much of a supply/demand imbalance to tip prices a lot one way or the other.
There's also a numbers problem for stocks, too. Older people have had a lifetime to accumulate assets. Younger people are working on accumulating them. Since our demographics are shifting toward more seniors, will the younger people who are accumulating assets be able to absorb them at the same rate the older people are disposing of them?
Jon