> Why would young people need to be vaccinated

The article answers that as: to prevent them from spreading the disease to others who are more vulnerable to a bad outcome from the disease. I am safer because I am vaccinated. I am also safer when the people who I meet are also vaccinated and are less likely to spread the disease.

Another good answer is that young people too can have bad outcomes from the disease. At a lower rate, but a lower rate times a large number of people is still a lot of bad outcomes.