I saw a clip from one of his recent rambling interviews (sorry, there’s been so many I can’t remember which one it was). 

He was asked whether someone his age was too old to be President. Obviously he said no, age is only a problem for Biden, it seems. 

He went on to say how he knew lots of people running businesses in their 80’s rather than passing them on to their kids because the kids weren’t as competent.

Given Trump’s tendency to make everything about himself, my guess is he’s referring to his own family here. The kids have realised he’s not going to give them control of his businesses, and will likely bankrupt the lot before he dies. So there’s no point in them hanging round.