I'm kind of like these other two, I'm 27 and go to 40+ sprint and midget races a year and have been going with my dad since I was 3. I think growing up with it makes a difference, but I think there are some other factors too.
I think one big factor is during your teens and early 20's, there's a lot of stuff to do like sports, school, partying, chasing girls, cars, just generally being out because you can, ect and that all cuts down on young people's time, money, and interest in going to the races. I'm a huge fan and go as much as a possibly can but as a teen I maybe went to 15 races a year for a few years. I've noticed as I've gotten older I see more and more people my age there with their young children. It seems that once people get out of the going out and partying stage and get married and have kids, they come back to the races and bring their kids with them.
As far as Silver Crown goes, I like the big cars and the history is awesome, but I enjoy the sprints and midgets a lot more so I rarely go to a Silver Crown race because usually I'm at another race somewhere.