Experience!

GT12's Fave Worst Pres Ever is discussed by George Will:

McCain, who at 72 is 22 years older than Alaskan statehood, is 27 years and six months older than his running mate, who was 8 when Joe Biden was elected to the Senate. But in 1856, James Buchanan, 65, was 29 years and eight months older than his running mate, John Breckinridge, who was 35. Buchanan could run with that stripling because Buchanan was the most qualified person to run for president, before or since.

At least he was if varied experience in high offices fully defines who is "qualified." But it does not.

Buchanan had been a five-term congressman, then ambassador to Russia, then a two-term senator, then secretary of state, then ambassador to Britain.

Buchanan then became perhaps the worst president.