Your link supports the quote I posted. The MLB has an average age of 53, and your data shows that they have more fans over 55+ than NASCAR does. Baseball is in worse shape than NASCAR right now. So is Golf. Golf and NASCAR are actually very similar in their rises to popularity and falls from the peak. Golf had Tiger Woods and now that he is basically gone, they are having their own issues, i.e. 63% of fans are over 55+.
I'm also not convinced you can get good data from people aged 2-17 pertaining to anything. Their likes and dislikes change too quickly.
It would be interesting to see the attendance records for those ages. Considering NASCAR has the highest average attendance for any sporting event, I'm willing to bet there are more kids going to NASCAR events than some of the other sports. With the new policies coming out for next year (Kids under 12 free for xfinity races, and $10 tickets to some cup races), I bet that attendance grows even bigger in the near future.
Other notes, NSACAR has the highest percentage of female fans, and the highest percentage of fans 35-54.
NASCAR will be around for a long time, just like golf and baseball. It will ebb and flow in fandom but always have a solid base.