As regards the United States:

There is no doubt that the population of Masons is aging. There was a huge increase in membership in almost all fraternal orders after World War II, including Masonry. This peaked at sometime in the late 50s. During the social turbulence and generational strains of the 60s and 70s, new membership fell off, with the result that by the 1980s, total membership was in sharp decline.

However, there are signs that membership has leveled out, or is gaining in some areas. In many lodges, there are a great number of 50-and-up members, and a number of 30-and-under members, with a gulf in between, representing where Baby Boomers would have been. Of course, we are speaking in broad generalities here-- there is no way to know the demographics of your local Lodge without asking one of its members.

The overall point is that Masonic membership, when talking on a national scale, has probably hit a stable membership base, after a huge surge and then fall in membership.

(Please submit information on other parts of the world)