Demographics certainly has a role, but it’s much more complex than what you state.
I have seen many a “baby boomer” business owner retire early, because they are sick and tired of the ever increasing struggle against their own gov’t’s policies, regulation, crony capitalism, taxes (if they are successful), and distortion of the playing field that ever more seems to benefit big companies. The generation before them simply seemed able to enjoy going on until they physically (or mentally) could not. Now, who the heck can enjoy it?
Conversely, many “young ‘uns” coming up today take a good look and say, “the heck with that”. If they don’t (take a close look) they usually fail.
But more significant, IMO, is that every single successful small to medium size business owner I know, who started out before 2000, says that in the present environment, they would not have succeeded. (I know only one who started after 2000, and he’s thinking of getting out.)
You raise another interesting point about younger folks. One of the reasons for the decline in the number of businesses isn't just demographics -- it's that people who retire today are being replaced by a lot of younger people who don't have the same maturity, capacity, drive, etc. to succeed in business.