Interesting question you pose. Although I had never seen it as an American/European distinction, you do have a point. I've always seen another distinction, and that's heavyweights vs the lower weights. Heavyweights tend to hang on too long, relying on their power which is the last thing to go. Smaller fighters just can't compete with the speed they no longer have. The biggest factors are obviously money, addiction to the adulation, and the stubborn self-belief that you can still do it. Add to that the examples provided by others in the profession... and you've got the perfect recipe for over-the-hill fighters hanging on too long.

Obviously there have been some well-known exceptions to the distinction you posed on both sides of the pond.