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.
A nomination does not mean that the republican party needs to back Trump. They could have simply chose someone else or even remove him from their ticket. It’s all in their control.
A primary is merely to check and see which way the political wind is blowing before the election. And nominations are not written in stone.