Well, some of them might have had other health problems beforehand that make the virus more dangerous to them. Even overweight is one of those problems, so an obese 20 year old is probably in more danger if they get covid than a healthy 50 year old. It could also be (note that this is just me guessing) that when some people get the virus, they think "nah, I'm young, it's no big deal" and don't take it easy, so it takes more of a toll on their body and eventually lands them in the hospital. And in some cases some young people might just have weaker immune systems even though they have nothing officially diagnosed.