Febrile seizures occur most frequently:

Essentially all febrile seizures occur in children between six months and five years of age. Indeed, most authors incorporate this age range into the definition of febrile convulsions, although some broaden the definition to between three months and six years. The peak age clearly is between six months and three years. In one study, the mean age was 23 months. One should be very suspicious of the diagnosis of a febrile seizure in a child much younger than six months or older than five years. Ref: Ropper A.H., Samuels M.A. (2009). Chapter 16. Epilepsy and Other Seizure Disorders. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor's Principles of Neurology, 9e.