This article from The Telegraph suggests that kids today are not as fit as the kids of 40 years ago:
While my son runs almost twice as fast as I do now, I sure do wish I had a record of my times from when I ran cross country in high school. Yes, it still wouldn't be a fair comparison given the gender difference, but it would still be interesting to see. I'd also love to see how my daughter compares to my time from then, despite the fact that she is a self-professed "non-runner."
Reading that article makes me glad I have active kids. Think I'll challenge them to become even more so.