She will celebrate her 69th birthday less than a month before Election Day 2016. As such, that will make her some 15 to 20+ years older than most likely Republican candidates, namely Ted Cruz, Scott Walker and Rand Paul. Not sure how she will handle the questions on if her health at 69 is good enough for the Presidency, especially given how much of a physical and emotional grind the Presidency can be.
There have been 10 presidents taking office at 60+ years of age.
President Reagan being the oldest at 69 years, 349 days.