I don't think the Sebring results are really meaningful. I say this because I observe a preponderance of older people involved in bicycle endurance events. I can't prove it, but I honestly don't think it's a matter of young people not being able to do this so much as not being interested, or maybe being more interested in regular racing. That is to say, having more old people in endurance cycling may be evidence OF age-related decline rather than evidence AGAINST it. Note that the original poster says he does fine in long-distance riding, it's the speed he's concerned with.

Even if you look at the population as a whole, I don't know that you can tell much. People tend to have kids and be busier around the house when they're there, and then get out more when the kids are gone. So if you can prove that the average 35-year-old is more or less fit than the average 20-year-old or average 50-year old, you're not necessarily comparing age-related effects so much as life-style issues.