China's population was very young because it had very high birthrates.
Step-by-step explanation:
Until the 1970's, China had one of the highest birthrates in the world. The country already had huge population and it was the most populated in the world by far, but its population was still rapidly on the rise.
From this population pyramid we can see several things. The birthrates were very high, contributing to high levels of young population. The middle aged people were twice less in number than the young ones. The average lifespan was below average, contributing to small percentage of old population.
While this contributed greatly to the economic rise of China, as it had enormous labor force, the laws that were implemented in this period now have a counter effect. Namely, the government prohibited the people to have more than one child, except with special permission, which now has made a problem as the country is facing the opposite problem, and aging population.