Blunt tools shouldn’t be used for complex real-world things. Other examples would be mandatory sentencing in courts (you should let the judge assess the particular situation), or things like “I don’t go out with guys under 160cm” (you should get to know him).
Just because someone is in their 70s, doesn’t mean they can’t lead. Not with any certainty. In some cases it does, but the political process should have the flexibility to deal with each case.
Look at Deng: he was mid-70s to mid-80s when he was in power and he improved the lives of 10⁸ or 10⁹ people.
This thread is getting sidetracked.
We can debate the various pros and cons of Deng in particular as a leader, but he generally has a good reputation (even the website you linked says so) and the point is about leaders in their 70s and 80s in general rather than him in particular.