https://www.populationpyramid.net/china/2019/

When WWII ended a couple countries had a baby boom. Australia and the US experienced a ton of births but the USSR didn't. They lost up to 75% of men born in certain years, and their demographics never had a recovery. The one thing that prevented other nations from completely balkinizing them from outside was the nuclear bombs they developed. They never had manpower and its obvious now.

The decline was made worse with birth control but accelerated after the 1990's. Right now they're going to get hit with a virus and the risk factors for its spread; old age, smoking, poor health, high urbanization, and mass transit are in unique confluence. About the only issue they don't have are tons of multigenerational families. They still have nukes.

Now you look back up to the link posted up top. China has a bizarre demographic profile. They have tons of young people, but a sex discrepancy from the one-child policy. Some people say they have 20 million more boys than girls, but maybe a few million girls were never officially counted. Its certain that a few million of their men will not he able to find wives in a society where homosexuality is taboo. A ton of their elderly have just died, at least 10 million victims and maybe upwards of 20 million.

Their demographic profile shows that they now can punch above their weight militarily.