I would expect the least developed nations to have a higher percentage of young people just because you're more likely to die of malaria or HIV or a tiger or something.
Interestingly, there's also a certain delay - even after fertility rates (children each woman will have over her lifetime by current trends) fall, birthrates (births each year per capita) take a while to fall, and even after birthrates fall population growth takes a while after that to fall as larger cohorts of young people replace smaller dying cohorts of old people. [https://en.wikipedia.org/wiki/Demographic_momentum].
What all this means is that on the one hand, population increase can be very hard to stop on a short-term basis, but also that current fast growth rates shouldn't be extrapolated too far into the future.