She's only 62, and will be about 70 in 2016. If McCain, Reagan, and Dole can win a primary at that age, so can she.
McCain and Dole lost the general elections in landslides and the same would have occured to Reagan had Carter not been a miserable failure and massive screwup.
So? The question was whether she'd run again, not whether she'd win. Back in 2002, lots of people thought McCain would never run again, as he'd be too old in 2008, but he did, at age 72. Clinton will be "only" 69 in 2016, and women live longer and have fewer health problems at that age. No reason why she couldn't run again.