This term was new to me although it has apparently been around in the US since the 1980s with reference to the aging population. There have been variations on the theme such as grey tsunami, grey wave, or age wave. Raising a different image is grey horde. We of course have our own grey nomads.
There are those who feel that terms like these are distinctly negative terms and arise from ageism. They would like more positive imagery for the increasing numbers of the elderly in our society.