One day I had a conversation with my English teacher. She was Filipino. When I told her about a 30 old years man I met, describing him as "a young man", she immediately pointed it out that "Thirty years old is not young at all".
I got it. In Japanese sense, 30s are still young. In some contexts, even 40s are deemed as young. They are withdrawn from the fact that the mean age of Japanese is over forty.
This is a graph describing median ages in each country. The data referred to Wikipedia. Population growth rates are added. The data refer to Wikipedia and OECD statistics in 2010 - 2011.
We know that Japan is one of the countries which aging has been developed in the world. I am 38 years old, but I belong to younger side in Japan. But other developed countries are in similar situations. Most developed countries are suffering from aging and low birth rate.
On the other hand, there are lots of young people in developing countries. When I visited Indonesia one year ago, I felt as if I was the oldest person there.
The meaning of the age differs in each nation. I have become to be conscious about the age of mine gradually. However, I hope that I keep my mind young and flexible forever.