Training racehorses requires stamina as well as skill and I suspect the commentator was impressed that he still had the stamina at 73, which is well past state pension age and an age when many older people are far less fit than they used to be and, if working, are not involved is such physically demanding occupations.
Similar comments have been made about a number of older tennis players this week. Remarks that they are or will soon be 40 and that it is remarkable that they still have the stamina to play long games, when even some younger players are showing signs of flagging during long matches.
Stamina and strength do decline with age, yet, remarkably, some people keep going long after others their age have slowed down.