There are trainers who take horses directly from the track and begin jump training immediately. I was in a Jimmy Wofford gymnastics clinic a few years ago where there was a rider who had gotten a horse from the track only the week before. He was a steeplechase trainer/rider and planned to run the horse in steeplechase races. This works well because steeplechase and hurdle horses are racehorses that run over fences.
If you want to event, do jumpers, hunt or just recreational jump, that approach is going to give you less than desirable results. You must first teach your new horse to be a riding horse and when he understands that, introduce jumping. This is not to say that some horses come off the track that never really were racehorses. It can be because of inadequate training but most of the time they just never embraced racing. You still must give them the basics.
Each horse is ready to learn to jump at a different time. Many times I introduce walking and then trotting over poles very soon after they come to the farm. If the poles elicit any excitement for the horse, I know that we’ll be walking over poles for a while before I show them a jump. One clue that the horse is not ready is if the he becomes excited when you start to jump him. If this happens you need to back track. Also be careful that you aren’t making a “big deal” out of it. If you are tense or excited, he will sense that and mirror it. Calm, easy going introductions work best. Many people just casually pop them over logs and natural obstacles while trail riding. I often wonder if horses take to this so much easier because most of the time the riders are more relaxed too. This is a good way to start.