Now here is more confusion.
I was told when I passed my test in KM at the age of 61, that my licence could not be renewed after I reached the age of 70.
Then during my efforts to renew my present licence, I was informed as it was no longer valid, I could sit another test and if I again passed, then my new licence would be valid until I reached the age of 70, and could be renewed after that by taking an annual medical.
Confusion abounds .

So indeed it is the age thing.
According to the present law no one will be able to drive after 70.
China has a lot of catch up to do with the whole age thing they are way behind in their legislation. Laws for driving are made as for chauffeurs and not for normal citizens.
Things keep on changing however. In China there are now important people getting old who want to be able to drive and there will be a push for a change in this because of that. So keep an eye out for possible changes.

I know the medicals are a bit of a joke but I know they keep the worst out. When I got my license there was a man in front of me in the cue (No privacy of course) who was as good as blind. He didn't pass despite his arguing that he only drives his 3 wheeler in the town where he lives and knows all the street there ;-)