<p><b>Purpose</b>: Epilepsy is a common neurological disease that places severe burdens on patients over a very long period. Current treatments mainly involve antiepileptic drugs, but these do not cure epilepsy. Improvements in treatment methods and targets have thus been awaited. Antiepileptic drugs given before the onset of epilepsy have been shown to exert beneficial effects on epilepsy prognosis in rodent and human studies. To establish preventive approaches to epilepsy, we organized a study team to clarify the mean age at onset of each epilepsy phenotype classified into genetic generalized epilepsy.</p><p><b>Methods</b>: Patients with genetic generalized epilepsy including epilepsy with generalized tonic-clinic seizures on awakening, juvenile myoclonic epilepsy, juvenile absence epilepsy and childhood absence epilepsy, who visited our institutes between 2014 to 2016 were studied. Demographic data from these patients and their offspring were analyzed. Offspring were regarded as a high-risk group because of high hereditability of epilepsy.</p><p><b>Results</b>: The study population comprised 154 patients (generalized tonic-clonic seizures on awakening, n=76; juvenile myoclonic epilepsy, n=52; juvenile absence epilepsy, n=17; childhood absence epilepsy, n=9) and 30 offspring. Mean (±standard deviation) age of patients was 27.78±9.69 years. Mean age at onset was 16.05±6.44 years, and mean duration of disease was 11.36±9.84 years. A high rate of multiple antiepileptic drug use and long duration of disease were found in all types of epilepsy studied.</p><p><b>Conclusion</b>: This preliminary study highlights the necessity for identification of offspring warranting early treatment together with optimal timing of such treatment.</p>