Tremayne compiled very interesting data on Presidential age. There have been 4 Presidents younger than Obama, and they were pretty good:

Obama would take office when he is about 47.5 years old which is only about 7 years less than average.  Here are the Presidents that were younger than Obama (alternatively, they had less life experience):

• Theodore Roosevelt, 42
• John Kennedy, 43
• Bill Clinton, 46
• Ulysses S. Grant, 46

Of course McCain would be the oldest first-term President at 72, and the five next oldest Presidents were pretty bad:

On the other end of the spectrum is Ronald Reagan who was elected at 69. Most experts agree he was showing signs of Altzheimers while still in office. Outside of Reagan, there have only been 4 other Presidents who took office within TEN years of McCain's current age (72 tomorrow):

• William Henry Harrison, 68
• Zachary Taylor, 64
• James Buchanan, 65
• George HW Bush, 64

Quite a distinguished crew there. Two who died not long into office and another routinely on the list of worst presidents. Somewhere in here is the makings of a good response to the age charge.