Teach Your Child to Read in 100 Easy Lessons (by Siegfried Engelmann, et al).
Virtually any educated adult can teach virtually any interested child to read at a comfortable 2nd grade level in just four to six short months (one 20-minute lesson a day, six days a week) using this classic phonics-based home learning tool. If desired, one can take up to a year to complete the 100 lessons (supplementing learning sessions with additional reading materials, extra writing exercises, etc.).
Instructions for the parent-teacher are crystal clear at every stage, and super easy to implement. Students should be able to recognize upper and lower case ABCs before starting, and it helps if to know the main sounds each letter makes, but this is unnecessary.
Using this system our daughter learned to read at age four, the proverbial child with her nose stuck in a book. She went on to be a happy, successful student, and a voracious life-long reader. Precocious children who want to learn to read could start even earlier. Most kids would probably do well to begin at four to five years of age.