The thing is - most 40+ gay guys I know are pretty settled, and pretty happy. Some are still single, and some aren't thrilled about the fact, but for the most part, they still live pretty happy lives.
From the vantage point of a younger-20-something who gets most of his "fun" going to the clubs and maybe hooking up with guys, they see this huge void. We're not going to clubs and hooking up, and guys aren't (often) flirting openly with us. So they think since this isn't happening, that we have nothing positive going on in our lives. But that's sort of like a six-year-old looking at those twenty-somethings, not seeing Gogurt or water park trips, and thinking that all the fun in life ends at puberty. Fact is - life changes, and the things you enjoy change. I'm the last to suggest that somebody put away their coloring books or club-going ways if that's really what they enjoy doing. But it helps to be open to new things, and to ditch as many preconceived notions as possible.
I'm over 40, and my life kicks so much ass, you can't even comprehend. How much ass? I still can't figure out an answer to your question. I guess "having my partner die too soon" would be an answer, but it's sort of a half-hearted one. It's not something I spend much time thinking about - I'm too busy enjoying my life.
Lex