Why is scarlet fever, caused by Streptococcus bacteria, more common in children than adults?

Adults are exposed to the toxin as a child, which initiates an immune response. B lymphocytes are produced and release antibodies which bind to the specific toxin and destroy it as part of the humoural immune response. This produces memory lymphocytes which possess antibodies specific to the scarlet fever toxin. In future toxin encounters, the memory lymphocytes activate a faster, more efficient immune response meaning the toxin is killed before scarlet fever can occur.