Complex sentence: Epilepsy is one of the most common serious neurological disorders affecting about 65 million people or about 1% of the population by age 20 and 3% of the population by age 75.
Split into simpler sentences: Epilepsy is one of the most common serious neurological disorders affecting about 65 million people. It affects 1% of the population by age 20 and 3% of the population by age 75.