Fitness in terms of endurance increases with age which is why the best marathon runners tend to be in their 30's. There was a very famous Finnish cross country skier, Eero Antero Mäntyranta, who never stopped skiing until he died. The Finnish national country ski team was out training one day and saw a man ahead and they struggled for miles to catch up to him. It turned out it was Mäntyranta, the Finnish champion who at that time was more than twice their ages.