'start' has a few aspects, mate. in most cases the dog is emotionally ready before it is physically ready. remember a dog which is to be heavier as adult can easily become susceptible to arthritis if too much excercise too young. in my opinion, for a bulldog, dogo etc I wouldn't dream of starting them before 10 or 11 months. if the dog you have in mind is a good proportion hound, then maybe 8 or 9 months. not before. any pup that is to work shouldn't be 'beasting' until almost a year old.
that said if you can have a younger pup tag along on a hunt, it does them good to be 'shown the ropes'