The United Kingdom Population Pyramid
From the population pyramid, it is clear that the amount of people in each age group up to the 60s are essentially the same. After the 60s, the amount of people slowly decrease.
This shows the U.K’s long life expectancy and good healthcare. In addition, the observation that there are less children then adults may suggest a large amount of immigration (likely due to wars in the Middle East), declining birth rates or both.
Overall, as a developed country, the U.K’s population pyramid looks the way it does due to access to advanced healthcare, declining birth rates and immigration.