Read something recently that says Japan as a nation is getting so old, that their medal count is going to continue to drop. The same article suggests at some point it will happen to China a bit as well with their restrictions on child-bearing.
Interesting theory. Japan has been aging for a while and I'm not sure if we're really seeing a drop in Japans performance. They've captured many medals at these Olympics so far.
As for China, their family planning policy has been in effect for over a generation and we're not seeing any effects sportwise. Not surprising, with a population of 1.3 billion and state funded sports programs, you basically have unlimited potential of sporting success.
Speaking of which, I have the impression that India is beginning to show themselves in more sports. They've won their first ever badminton medal, were active in shooting and archery and I saw Indian athletes at the discuss and 10k for women.
There's plenty of untapped sporting potential in India. In Africa too, by the way.