Final answer:
Japan has the oldest average population in East Asia.
Step-by-step explanation:
The country with the oldest average population in East Asia is Japan (B).
This can be determined by looking at the information provided. While China is the most populous country in East Asia, Japan has the oldest average population. This is because Japan has a higher proportion of elderly individuals compared to other countries in the region.
Based on historical and cultural factors, Japan has experienced a rapid aging of its population. The country has one of the highest life expectancies in the world and a declining birth rate. These factors contribute to an older average population compared to other countries in East Asia.