I'm sorry if that sounds like a cop-out, but that's the way it is. OR it may not. It all depends. It all depends on how come the child learned to read ahead of most other children. If a youngster learns to read early because reading comes easily and naturally, that is one thing.
Some children have the kind of intelligence that makes it easy for words and letters to sink in.
Some come from homes where mother and father are real bookworms - that can be a help. Heredity plays a part in early reading, too. No one has strained, no one has struggled. It was no big deal. When early reading comes about in this natural way, a youngster gains - there is no getting away from it.