As I mentioned earlier, epilepsy can occur at any time of life. Though we see it in elementary school children, we also see it in the elderly. I got that information from a neurologist. It is also quite common in the elderly because of different problems. However, it can happen at any age. Unfortunately I do not have exact percentages. But you see it most in elementary school children.
Epilepsy is an entire spectrum of disorders. There are benign epilepsies, like Rolandic epilepsy, that will completely disappear at puberty. These days, a first seizure is not even treated. Seizures must be recurrent before we talk about epilepsy. The acute syndromes on the other end of the spectrum will appear very early in infancy and will be extremely disruptive. Those epilepsies are difficult to treat and can cause the child a great deal of distress.