The correct statement regarding demographic trends around the world is:
The world’s population is getting older on average because of long life expectancy in some places.
As life expectancy increases in many regions, particularly in developed countries, the average age of the population tends to rise. This is contrasted with areas experiencing high fertility rates, where populations may be younger on average. However, globally, the trend is moving towards an older population due to declining birth rates and increasing longevity.
The statement about the world’s population declining isn't entirely accurate at this point, as the global population is still growing, although some countries are experiencing declines.
The statement about developed countries having longer life expectancy due to high fertility rates is misleading; higher life expectancy is generally associated with lower fertility rates.
Lastly, while some regions have high fertility rates leading to younger populations, this does not mean the overall global population is getting younger. The global trend leans towards an aging population.