At what age does epilepsy start? Is it hereditary?

Any age. Epilepsy may start in early childhood and in the elderly. There are different kinds of epilepsy, some of them are idiopathic (cause unknown), whereas others may have a plausible cause such as head injury. Epilepsy is not hereditary but may have increased incidence in relatives of people with epilepsy.
Good question. Many cases appear to relate to genetic tendency that runs in families. Increasing evidence supports genetic links to many forms of epilepsy. Some are present at birth, many present childhood and puberty. Many adult forms are acquired due to trauma, infections, stroke, tumors, drug use, etc. Very complex disorder and actually very important. 5% of children with seizures; 1.5 to 2% adults.