I really don't think age matters one way or another as much as one, a person's qualifications and two their health in regards to how they can handle their job.
I agree, age is unimportant especially compared to the vast array of experience and knowledge accumulated by Hillary Clinton. If she is healthy I do not foresee a problem, especially since science has made such strides in medical research.....people are living longer and are more actice. Ronald Reagan was almost 70 years old when he was first elected to the US presidency.....A lot has to do with the individual personal stamina and makeup; Hillary's determination to be president of the US is so overwhelming that only someone unforeseen at this time could unseat her.
The only force making age a problem for Hillary is the GOP....and boy have they started their tirade about how "old and tired she looks." Clinton's poll ratings are in the 60s, far ahead of any possible GOP candidates and VP Biden. It seems more Americans want her to run than those who don't.
PS Even though Hillary seem like the inevitable Democratic Presidential candidate, there is Elizabeth Warren, a a remarkable or exceptional person....a phenomenon really, who could very well give Hillary a scare, although Warren says emphatically, she's not planning a run for the presidency.