In a YouGov poll conducted between July 25-29, 59 percent of Republicans surveyed said they would prefer a president under the age of 75. Trump celebrated his 78th birthday in June. Forty percent said they had no preference, and just two percent said they would prefer a president over the age of 75.
The Democrats still haven’t had their convention so they still can make whatever chabge they want. If Trump withdraws, the Republican party as a whole would be withdrawing because they already had their convention and chose their representative. Another Democrat can run against Kamala up until the convention, but once the Democratic party chooses their representative, they would also be locked in.
The DNC and RNC aren’t legally bound by the rules they made for themselves. There have been lawsuits in the last 8 years that have established that. They can both do what ever the fuck they want as long as they don’t piss off the really big donors.