Hypothetically, but parties are also incredibly weak right now.
In California, we run an Open Primary system for every partisan office outside the POTUS so parties can’t even guarantee they’ll get a candidate into the general election even if they are otherwise eligible to put nominees forward and sometimes it’s Democrat vs Democrat (this has been happening for US Senate elections) or in some legislative districts Republican vs Republican.
Then coming from the opposite direction, you had a complete outsider in Trump effectively takeover the Republican Party in 2016 and he’s still holding the reins in 2024. Bernie Sanders almost managed something similar with the Democratic Party and they just barely held strong enough to keep that from happening. Twice.
Frankly political parties are incredibly unlikely to come together to impose any restrictions that would impose any kind of extra burden on their elder members, let alone age-gate them.
I missed the part where you explain how Bernie Sanders is a meaningful analog to Donald Trump, such that his primary defeats should be hailed as “holding strong” against an insurgent candidate?
Well the unknown variable is whether like Trump he would prove to be more popular than the party he ran under after a successful Presidential election effectively taking it over so you got me there; but they were in 2016 both outsiders who came into their parties to run for President under an already successful Party-brand and made a helluva run, one more successfully than the other.
Either way, neither of them should have gotten as far as they did.
That can't stop an elderly candidate from running third party to spite the eligibility rules, but that's already the case.