It seems to me to be a function of available time and degree of enjoyment derived. I have weekends available and while I have many pursuits, Homebrewing has risen to be one of my most favorite - I prefer it over golf, but probably just behind fishing, but fishing involves travel (I like to fish at certain Northern Wisconsin lakes, so I go to those places maybe 5-6 long weekends per year). So, Homebrewing is what fills one of my weekend days usually at least twice a month.
Dave is right, though. Only a few young adults are willing to carve out the time to home brew (for example, my son grew up around my Homebrewing and loves my beers, but he and his buddies prefer to go golf and then to one of several microbrew/brewpubs where he lives. He says when he gets a house and a garage he may pick it up as a hobby, but for now he has little space and too many other options availablefor his time and money.