Age really doesn't matter - consult your vet before starting to train on the spring pole. If the dog is very young you will have to construct it low to the ground - the dog should always be able to touch their back feet to the ground when working on a spring pole.
Be careful when introducing dogs to spring poles, and adjust it to your pet's size and height. This activity is likely safe for dogs ages six months and older.
It is almost never too early to start a pitbull on a spring pole. Pitbulls are strong even from a very early age. You don't need to worry about hurting your dog.