Here I'd like to note that incumbents usually do pretty well at getting re-elected. A two-term McCain presidency would have him going into it at 76-77. That's getting up there.
Reagan was 69 when he first got elected to the White house. If you go by that, then RPM's right. If you go by re-election, Warspawn's correct; We can't assume that McCain would get/persue re-election, but if you count the fact that most president's do get a second term, then McCain will potentially be the oldest by a fair margin.
Wilson FDR Ike Reagan Clinton (5) 2 full term presidents in the 20th century.
McKinley, TR, Taft, Harding, Cooledge, Truman, JFK, Johnson, Nixon*, Ford, Carter, GHWBush, (12) less than 2 full term presidents in the 20th century.
Since George Washington, the average time in the white house is only 5 years, 1 month, 3 1/2 days
*was re-elected