Sure that's a factor, but how do you account for Ecuador with a median age of 28 and a higher per capita death rate than the UK or USA? Ecuador has less healthcare capacity but that can't account for the whole difference. Conversely, Sweden has a median age of 41 and a lower death rate.
It's a statistical association, not a rule. I can't say specifically why those two countries have had the results that they have, but the age dependence holds both at the individual and country levels overall. That also doesn't mean that you can't find a 100 year old survivor and a 20 year old that dies from it.
8-10 years difference in median age is an enormous difference in the proportion of 70+ in the population, and an even bigger difference in 80+. This is a huge part of the reason that you haven't seen the death toll be nearly as high in developing countries as in developed countries. In general, developing countries have a much younger population.