1 child policy. The population pyramid is inverted, so the average age is increasing.
The 1 child policy is leaving China with a low birthrate and hence an increasing smaller population of young people. The problem is further compounded when Mao encouraged Chinese people to have as many children as possible, which lead to ~6 childbirths per women in the 1960's.
This means that, at some point, there will be too many old people to be supported by too little young people. Japan is facing similar population issue for example.
Now, with the population index slowly inverting, China is running out of time to grow rich before it grows old. Otherwise, its social infrastructure will at some point become overburdened by old people who require care/safety net and not enough young people to to provide that care/safety net (either directly as children taking their aging parents or indirectly by paying taxes).
Also, China needs to figure out how it can overcome the middle income trap. Otherwise, its growth potential is severely limited, and could be more fuel being added to the rapidly aging population problem.