Bulldogs are alittle different, their teeth don't set so you run a good chance of screwing his mouth up starting him that young.
As far as bay dogs go, it depends on the bloodline, but outta the 100 or so people claiming they got a puppy finding and baying at 4-5 months old, I doubt one or two actually have it.
X2...by all means show young cd's pigs and dogs baying to get their interest up and I wouldn't mind letting them start catching small shoats around 12 months but wouldn't really put them on pigs for a few months after that. There teeth don't fully set until 14-16 months and I don't want to mess up the dogs mouth. That being said I know several dogs started earlier that made great cd's but most of those don't have much left for teeth.