It seems likely that Candidate Obama's youth helped him against Senator McCain, who was 72 when he ran for president in 2008. In 2016, Hillary will be 69 -- younger than McCain, but a striking 22 years older than the average Republican candidate:
Then, there's Clinton's health and age. She'd be 69 by election day next year compared to a field of major Republican candidates with fresh faces who average 50 including Romney and only 47 without him.
Whatever you think of it, it's hard to imagine that age won't be a factor in the election.