I think we need to either have term limits or change campaign funding to public funding, so incumbents don't have an unfair advantage. Until just recently, the two Senators in my state (one a Republican, the other a Democrat) had each been in office for over thirty years. The Democrat finally retired or he might still be in office. The last time our Republican senator was up for re-election, his opponent challenged him to a debate, and he refused. Mr. Incumbent has every advantage: he's well-known and well-liked, thanks to the commercials that PAC money paid for, and debating the issues would only allow his opponent to illustrate where Mr. Incumbent made bad choices or favored the corporations who fund his campaign over the majority of voters. It would also allow Mr. or Ms. Challenger to introduce him/herself and his/her ideas, which voters might favor over Mr. Incumbent's.
The problem isn't a need for a retirement age. The problem is that being in political office shouldn't be a permanent job that you retire from. It should be a temporary job with a lot of turnover.