As for China, this is a result of the One Child Policy. They are trying to reverse it now because of the aging population but it seems people are not interested in having more children.
The Chinese population is around 1.4 billion, and despite the one-child policy, the population is still growing. Analysts expect the Chinese population to grow until 2030. Without the one-child policy, the situation could have been disastrous.
Yes it's still growing but but elderly segment is growing as well. More elderly means more people on pensions, plus healthcare, etc. They would have not relaxed the policy if they think they'll still have a population boom for decades.
About the policy, we all probably heard horror stories of its implementation. Still, some would argue that it was necessary, like the Black Death in Europe.