Des, one to two years old is what I go by. Any dog under a year old is just an immature puppy.
Yes, there are dogs that start early. I've seen way too many dogs culled, ruined and even killed on hogs because their owners pushed to hard and expected to much from them.
I don't even start a young dog in the woods anymore until they are a year old. Let them be a puppy the first year. Get a handle on them, socialize them, show them what a hog is... then haul them to the woods with the older dogs.
I couldn't agree more mike ,I don't like starting a dog before its at least 10 -12 months old ,but once started I never leave them at home & get as many pigs into them as I can while they are still young.