Fifty-nine percent of Republican voters would prefer a president under the age of 75, according to a YouGov poll.
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.
Trump (recently) received the nomination from his party, while Biden dropped out before the DNC. I could be wrong but I think it's a little more problematic for him to drop out, although perhaps that's just related to RNC self-imposed rules and not any actual laws.
IMO doesn't matter whether they can or can't, they already attacked Biden for doing it as if he actually was nominated, so that fully puts them into hypocrite land if they do.