How young can a horse be to put horseshoes on the back feet?

Need more info. What is the reason you would be considering to put shoes on a young horse? If they need it for corrective issues-the youngest we shod at our barn was 2 months and he had to have shoes for just over a year and then went barefoot. He was shod again when he went into training at the age of four. He was a cutting horse by the way.