Term-limits are blatantly anti-democratic and age limits are clumsy, but a cognitive evaluation and probably an MRI would be good for rooting out cases of cognitive decline.

There is an informal age limit in China and Xi is still below it, though just barely. I'm curious if he'll go for another term after crossing it. I think he understands that he needs to retire sometime -- no one wants to become a late '60s Mao.