Why are kids under 5 and adults over 60 at risk for diphtheria?

Diphtheria. Children under the age of 5 , most specifically less than 6 months are most vulnerable due to lack of antibodies to counteract the disease. This is the most important reason why these infants should start receiving the vaccine as early as 6 weeks of age.. The risk for diphtheria in adults over 60 is due to waning immunity. This is why receiving regular boosters every 10 years is important.