I wonder about this myself and would love an informed answer.

My guess, is that because of constant "lawn mowing" older Gazans were killed off at a disproportionate rate, massive poverty tends to increase birth rates, family planning supplies or medications were hard to come by, constant "lawn mowing" convinced people to have more children in the hopes that some of them survive, and pollution or other environmental factors causing premature deaths.

That's just a guess though, and I hope someone with more knowledge chimes in.

they were not fucking more 19 years ago AFAIK, palestinians just almost never survive to adulthood because of concentration camp living conditions and constant IOF bombing.

median is a cummulative statistic. it doesn't mean there are more 19 year olds -- it means half of all people are below the age of 19. my understanding is that it's so low because the likelihood of surviving to older ages are poor.