I was speaking with our pediatrician when our kids were young and this was 45 or more years ago and I recall he said something about how tough and resilient young are. I guess I took this to mean they their immunity is greater.
If I am wrong perhaps I either misheard him or came to the wrong conclusion.
I think structurally they could be considered tougher - bones aren’t fully hardened, so they are more likely to bend than to break, but their immune systems take years to develop.