"Star" "crystal" "indigo" and all those "children" are nothing but a fancy modern terms for an old soul. The older a soul is, the weaker hold a physical body will have on the mind. So they act more mature, wiser, etc.
An old soul's mind is still very much connected with their spiritual existence.
When a typical soul is incarnated it is only aware of the existence it has had from birth to present day.
But an old soul is not cut off as easily and knowledge slips through, causing others to take notice. Old souls are not as commonly incarnated because they no longer need the lessons of life, therefore they are here by choice, for healing or to revise their ways. They don't need that complete cut off of memory. Their ways are so set through age that I don't believe it is even possible to cut off their memories through any means.
No, you won't find this in any books. It's common sense to me =)