I wouldn’t put it past Jr to be using these attacks for personal tactical advantage. I see two possible scenarios.

He’s said to have political aspirations of his own. If Biden is too old now, then Trump is too old in 2024, as you say. So who do Republicans vote for? A vote for Jr is the same as a vote for Sr and will keep Trumpism alive. Yay! Same way as a vote for Lurlene Wallace was a vote for George, until he could legally run again way back when. They work it as a stepping stone into a seat of power.

Or….

Revenge. He’s passive aggressively trying to tank his dad’s chances at reelection to get back at him for promoting and endorsing Ivanka politically over him when Sr knew Jr had political aspirations and Ivanka does not. We all know how petty and vindictive Jr is. He really is a chip off the old block.