If we could travel back in time 150 years or so, what would we notice as we stepped out of our time machine? Of course, there would be no electronic devices or things made out of plastic. There would be no radio, no TV, and no planes in the sky. However, perhaps the biggest change from the world we know would be the large number of children that we would see. Since that time, especially in developed countries, the average age of people has increased all over the world. In developed countries, the median age of a country’s population - the age at which there are the same numbers of older and younger people - was 29 in 1950 and 37 in 2000; by 2050, it will be close to 50.