My youngest son started having them when he was about 18 months old. His pediatrician told us he thought he would grow out of them by the time he was 5 or 6, but he's 8.5 now, and still has them. They aren't fun, but at least they are worse on the parents than the child. The child has no recollection of having a night terror. We just hold him when he's having one, and keep him from hurting himself or anyone else (he flails his arms and legs around everywhere, and we worry that he's going to hurt himself).
The only thing that seems to trigger them is being over-tired, but he gets tons of sleep, so sometimes that doesn't seem to be it, either. We thought he was finished with night terrors at one point because he went about 7 months without having one, but then he went through a really bad spell of having them again.
My niece had them, too, but she has outgrown them. My mom said that I had them, also, but it was before they knew what to call them. My oldest son had/has nightmares, but no night terrors.
((hugs)) I know it's difficult and frightening to see your child going through this.