As medical practices continue to improve, and as we learn more about diseases and aging, we could expect to see eighty year old candidates much more frequently. But we can also expect some younger Kennedy-Clinton-Obama types too, particularly among Democrats. I was curious, so I added the ages of candidates since 1960. The average age of Democratic candidates has been about 55. The average age of Republicans has been about 64. Trump 2020 and Biden are both the oldest in their respective parties. Bush 43 in 2000 was the youngest Republican at 54. Kennedy was the youngest Democrat at 43.
I'm fairly sure Nixon was in his 40s in 1960.