In my experience, young riders in great shape will outperform older riders in great shape. The equalizer is that most young riders aren't in great shape, they have lots of other distractions like work, dating, kids, partying, etc. As an older rider, if I eat healthy and train hard I can compensate to a certain degree for an increasing age. I'm 64, and if I want to use age as an excuse for being dropped on a group ride, I certainly could...but I don't, not because I wouldn't like to, but because it doesn't help. Today is the youngest I will ever be, and I'll train as hard and ride as fast as I can because I enjoy it. When I get dropped it is because the other riders are stronger than me, no matter what their age. Health problems can be a game changer, and fortunately I haven't experienced any...yet.