It's interesting to note that the oldest Republican in the 2016 field is Jeb Bush. (We only included the top-tier of candidates in current polling.) Bush will be nearly as old as his father was 28 years prior. His brother George was much younger, having been inaugurated 14 years ago. When George W. took office, he was as old as Rand Paul would be if Paul won today.
That timespan issue comes up with Clinton, too. When Bill Clinton was inaugurated, he was third-youngest, and the second-youngest ever to be elected. (Teddy Roosevelt became president when William McKinley was assassinated.) Clinton's inauguration was 24 years before the 2017 event. He could enter the White House first as one of the youngest presidents and again as one of the oldest first spouses. (There's another first there, too, of course.)
We're revisiting this in part because of analysis from Geoffrey Skelley at Sabato's Crystal Ball. He pointed out that, first, the likelihood was small that the difference in age between the Democratic and Republican nominees would be anything unusual -- or, at least, any bigger than in 2008; John McCain was 25 years older than Barack Obama, the widest spread. And, second, he noted that being younger doesn't offer much of an advantage for Republicans -- particularly in the case of Clinton, who's expected to do well among younger voters.
The youngest possible president in 2017 is Marco Rubio (currently 43 years old). He, too, has a solid answer for those worried about his youth.