Everyone who has not already been infected is at risk of infection, but not everyone who is infected becomes ill.
HFMD occurs mainly in children under 10 years old, but most commonly in children younger than 5 years of age. Younger children tend to have worse symptoms.
Children are more likely to be susceptible to infection and illness from these viruses, because they are less likely than adults to have antibodies and be immune from previous exposures to them. Most adults are immune, but cases in adolescents and adults are not unusual.