Well that's not entirely true is it unless we are living in the land of wishful thinking!
While there are certainly guys who are a better fuck at 55 than other guys who are very much younger that doesn't mean that age doesn't have any effect on performance. You are not comparing like with like. Some guys are just better lovers than others for a variety of reasons.
My hubby is 66 and still fucks me well but he'd admit himself that he doesn't have the same stamina he had 20 years ago or the ability to repeat as often as he could. But hes still a better fuck than some younger guys I've had.
So of course age makes a difference. If it didn't we'd have 70 year olds competing in the 100m!
But experience, skill and fitness (not to mention cock size) all make a difference too!