I think part of the issue is that gaming in general is concerned with adventure and excitement, and Western culture doesn't see people in their 40s+ doing much of that. We're assumed to be running to soccer practice, paying our bills, and trying to figure out what to cook for dinner. And if we do look for adventure, game companies seem to think, we look back to our 20s when we weren't as tied down. The age-range of game characters really is the assumed golden age of excitement and freedom.
Personally, I'm pretty happy being in my 40s. I feel like I'm less of an idiot who took way too many chances. I'm also not ready to "pass the torch" to my kids. I'd like to adventure WITH my children, personally, or find some other ways to explore mystery in my life. But, again, that's not how our culture (or, well, at least my culture. I'm from the US) thinks of people in their mid-age.