I've been enjoying and pondering this thread, and while I think that there are a lot of good reasons given why there are fewer "younger" sailors (and while I'm not quite 40, that still makes me generally a younger sailor).
I wholly agree about the economics of it--time and money are both scarce for those of us who chose a "conventional" path (job+family) during the post-educational period.
But another factor that I think gets ignored is the demographics of the areas where people live and sail. In my harbor (Belmont, four miles north of the Chicago Loop/Central Business District) there are lots of younger boaters, both sail and power.
Conversely, when I've been up to the harbors in the northern suburbs, the docks and clubhouses are filled with more traditional "old salts." And I won't even begin to discuss places like Florida.
While this might seem hopeful, I think this may be anomalous for a few reasons. First, the Near North Side where I live is also "the" area for younger people to live (they just have to tolerate me

). Second, it's an extremely dense urban area. As a result, there's a much lower rate of car ownership here than in most places--my family does not own a car and we get along just fine--which frees up a lot of cash for expensive hobbies. Third, there's a lot more free time since many more of the people tend to be in the pre-family stage of their lives. And, finally, a lot of the people here, growing up and living by the lake, either had exposure to sailing or the dream of sailing after watching the sails out on the lake their whole lives.
Normally, I'd agree that many people 30's and younger have never been exposed to sailing, have no idea if they like it or not, or how much it does or doesn't cost. For example, when people ask me how much my mooring is, I tell them the price per season and their response is almost universally, "Per month!?" Throw in the seemingly abstract artistry of it all--the terminology, the mystery of sail trim, and the latency & patience inherent in maneuvering something that weighs several thousand pounds, and it overwhelms people so that they don't even know where to start.
Now, though, I think the challenge becomes looking at what can be done about it--this is as much a campaign to correct misperceptions (e.g. costs, "sailing is for old people," "all sailors are racers," etc.) as getting people's attention and awareness.
One final observation: On any given weekend, 90%+ of the boats out on Lake Michigan are sailboats (I've counted). Head into the anchorages, however, and it's inverted, with the vast majority of boats being powerboats rafted up into big floating parties. Powerboaters seem to buy boats because they want a floating condo or a status symbol; sailors buy boats because they love to sail.
Do others observe similar demographic skews one way or the other where they live?