Yasuhiro Nakasone, PM of Japan 1982–1987, has some potential. Despite his stepping down in the 80s, he remained a member of the Diet until 2004— and only because the then-current LDP leader was pushing out the "old guard". By 2010, Wikipedia says he was "aware of his status as one of the few leaders revered across Japan's suddenly fractured political landscape" and "the country's most revered elder statesman"… which both sound like reputation-burnishing lines to me (no citations given for these quotes!) but still gives us something to work with.
Let's say that Nakasone isn't dropped from the LDP list in 2004— maybe he more effectively pushes back against his impending deselection and is granted a reprieve, Koizumi doesn't resort to deselecting the old guard, or Koizumi just isn't leader to begin with… the point is, he's re-elected in 2004 and onwards. After the DPJ comes into office and the LDP is in disarray, Nakasone's sharp criticism of the government while still owning up to his own party's failings sees him return to the spotlight and become a key opposition figure; he then wins the LDP leadership in 2012, and then becomes PM— again— later that year. Assuming he can hold onto the office, he would fulfill this challenge in May 2018.
If sub-national governments count,
Ken Hechler could be a possibility. He left office as West Virginia's secretary of state when he was 85— but didn't retire from politics, making a few more unsuccessful runs afterwards. If we change his post-secretary races, and have them go his way… it seems possible to manoeuvre him into the governor's office for the 2012 election— where he would hit triple digits before the end of the term.