I, too, have been surprised at all the younger people that are into the show. Most of the people that I know who watch it are older than I am (parents, aunt and uncle, etc). But, I think it's great - there seem to be very few shows out there that appeal to such a wide audience. CSI seems to have something for everyone....drama, humor, the hint of relationships, a puzzle to figure out, great characters with great chemistry, actors that appeal to everyone......
I guess to get 20-some million people watching the show each week, all the age groups are tuning in.