That China's population is aging now doesn't mean it will in the future. You have to remember the birth rate in China was very high. But now the average Chinese woman understandably doesn't want four children, so the population is shrinking because due to the higher fertility period there are a lot of older people. But when they have died out, every generation will be from a generation with fewer children, so there will be a more even spread of age demographics.
Also capitalism requires more people because of the need for higher profits, so they push for higher birthrates. Once birth rates fall, which is inevitable in capitalism, capitalists fear fewer workers to exploit and fewer profits. But in socialism the economy isn't based on exploiting as many people as possible for the highest profits so less emphasis on population growth is needed. China is able to deal with aging better because of this.
Besides even a depopulated China would be one of the most populated countries in the world.