Clinton's age is irrelevant, as others have said, it's her health that's important. Age shouldn't be a barrier in running for President - as someone who's a fan of Reagan, and as someone who backed Paul in the 2012 primary, it would be extremely hypocritical for me to grill Clinton over her age.
Age and health are tied together. You're closer to the grave at 68 than at 60.
I don't see this being an issue at all in 2016. You just don't criticize the first woman nominee on anything that could be tied to her appearance and have your political career survive. If she is personally concerned about her health, it might be her plan from the beginning not to run again in 2020.
A handful of potential nominees won't have to worry about a political career (Well, mainly Jeb.) And there are a lot of surrogates who could make mean comments, while distancing themselves from the Republican party's nominee.
But I don't see questioning someone's age as that significant.