However, had Senator John McCain been elected president, he would have been the oldest president to be inaugurated at the age of 72 years old. revolution | College Teacher | (Level 1) Valedictorian. Posted on. (Answer #8). The oldest US. President to be elected into the White House, and assumed office was Ronald Reagan.His age around that time was 69 years and 349 days old. He survived an assasination attempt and when he finished his second term as President, he was already 77 years old, so old but still very strong.owever, had Senator John McCain been elected president, he would have been the oldest president to be inaugurated at the age of 72 years old. revolution | College Teacher | (Level 1) Valedictorian. Posted on. (Answer #8). The oldest US. President to be elected into the White House, and assumed office was Ronald Reagan.