Its called the 'Seven Up, Eight Down rule'

Reference on wiki:

'This has been known as the rule of qi-shang, ba-xia (七上八下; lit. "seven up, eight down"), referring to the fact that if a PSC member is 68 or older at the time of a party congress, he must retire, but if he is 67 or younger, he may still enter the committee.'

Its probably likely Xi will retire at his next review. This thing is just a general non-written rule that happens, the vast majority of people retire around age 67-70 for the CPC. They are then usually used in supportive roles to advise people in their seat on what was done in the past. I think they generally just eyeball it, if someone seems to have issues with functioning in their role they suggest retirement.