well, i am convinced that at a certain age people should step down (or be removed) from positions where they have direct, first-line control over the future of countries, corporations or any other organization (eg a church). No problem with advisory roles to contribute the wealth of their experience and give reasoned, balanced feedback to younger ones running the show.
what age? well "legal retirement" age in most of europe was and often still is about 65. While there is obviously an amazing amount of individual variation, i'd consider 65 to mac. 70 a very reasonable threshold. even the catholic church has a defined retirement age for bishops (75), but absurdly not for the position of its supreme commander in chief (pope).
mandatory retirement age should definitely apply to any political/public office from head of state/president to Superior Court Judges etc, and church leaders including the pope and to CEOs and managing/board directors of public (stock market listed) corporations.
No disrespect, but simply from a certain age onwards, most people are too much concerned to preserve some status quo and power structures from the past, rather than actively looking for new solutions and shaping the future. As witnessed at ultra-conservative Canon.