As an adult, can I catch fifth disease?

Yes . Most common in the child but can be seen in adults.
Certainly yes. Erythema infectiosum, commonly known as fifth disease, is most often a trivial childhood illness. However, if an adult who never had it before catches this viral illness, it can be way more serious. Prolonged arthritis is one of well known complications. Infected pregnant woman passes the virus to her baby resulting in hydrops fetalis/congenital anemia.