Many dog owners use retractable leashes and love them. A retractable leash should give you and your dog the ability to loose leash walk while also being able to stop the dog in his tracks while you catch up very quickly. When used correctly, a retractable leash should not pull on your dog but just give you the loose leash between the two of you and the ability to close that very quickly should the need arise. With a retractable leash, your dog shouldn't necessarily know there is a leash between the two of you. Retractable leashes are meant to be kept loose and not tight. They are not meant for pulling the dog towards you. If your dog slows down or walks back towards you, the retractable leash should retract inside the handle. This is very convenient if you don't want the slack in between you and your dog to get in your way.
Training your dog to walk on a retractable leash will begin with teaching your dog how to loose leash walk. As mentioned above, your dog shouldn't really know there is a leash between you and him. A retractable leash just gathers excess leash into the handle rather than leave it lying on the ground or leaving you holding the excess leash in your hands. You can teach any dog at any age how to walk on a retractable leash. The key to using a retractable leash is knowing how to train your dog to loose leash walk with you. If you have a dog who can overpower you and pull you along on a regular leash, loose leash walking on a retractable leash is probably not going to happen with your particular dog. Be sure you understand how well trained your dog is before you introduce a retractable leash. If your dog is strong enough to pull from you, he can pull the retractable leash handle right out of your hand, causing himself injury or someone in between you two.
To teach your dog to walk on a retractable leash, you will obviously need a retractable leash. But you will also need some other fun items such as high-value treats like hot dogs or cheese. Have some patience and go on short walks close to home or even start in your backyard when you are first teaching your dog to walk on a retractable leash. Understand how the retractable leash works when your dog pulls away from you as well as when your dog comes back towards you, so you know what you are supposed to be doing with the leash as your dog moves far from you or nearer to you.