Why the Spanish Flu had a great impact on the Young?

MOST flues have a strong impact on the young.  The Chairman Xi Bat Flu is an exception and nobody is yet sure why. 

The pathology of viruses is different depending on their strain and the previous history of the population. Older people had already been exposed to influenza viruses from earlier pandemics and epidemics - but anyone born in the last decade of the 19th century would not have any natural immunity. The other factor here is that health services were so poor at the time that the chances of people making it into old age with heart conditions or the like was low.

100 years later there is still no definitive answer.  There is speculation that 20-40 year olds were hit so hard because it wasn't the flu killing them, it was their immune systems' response to the virus.  The 'Russian flu' of 1889-1890 killed over a million people around the world and infected tens of millions more. The speculation is that youngsters between infancy and ten years old who were exposed to it acquired antibodies, and that when they caught the 1918-19 bug, those antibodies remained buried and triggered a hyper-response in the immune system that their bodies could not handle.

The Spanish flue was also deadly to those 5 and younger, something else not normally seen. It is possible that young mothers who carried the antibodies from the 1889-1890 flu passed those antibodies along to their infants and the same response was seen.

But again, it's speculation. No one really knows why the virus was so deadly to age groups that are not normally hit so hard.