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.
Having a one child policy can kill chinas economy in the future trust me. However that is being irrelevant to the topic. Japan had 2 child policy and been working this for a while so their current population is quite healthy. Yet this idea as well can stress out their child because they can focus o their children with regards to everything