To answer Jake's question, I was 10 y/o when I became a Monkey Island junkie. And like Jake, I still like those games.
Figures.
It's an interesting topic, though. Somehow Monkey Island, Day of the Tentacle, etc. were the game equivalents of Indiana Jones and Star Wars movies - something that looked like it was meant for a more mature audience yet found its most devoted fans in the boys entering their teen years. I think the "speaking down to" issue is important and that's something that these games avoided. Why? I dunno, perhaps because they were created by game enthusiasts who wanted to do something they loved and liked to see on the computer screen, rather than studying an assumed market segment and demographics of what might be marketable.
These thoughts just from the top of my head, I'll give it some time and get back if I think of something profound.
