Why does fifth disease mainly effect children between 3 and 5?

Not sure. I believe it has to do with how there immune system works with the virus.
They're not immune. Parvovirus b19 infection is common in children. The outbreaks occur at schools and other places where children are together. During their infections, only some people get symptoms, while others seem normal, not ill. Regardless of whether they have symptoms, they become immune after infections, so they don't get the disease as older teens or adults. About 50% of adults have been infected.