The sooner the better for me, it sure doesn't hurt to carry them with you. I wouldn't expect a pup to hang with the big dogs until he's been on a LOT of practice hogs first to educate him.
For my young dogs, once or twice a mnth, from the age of about 3mnths on, they get to bay a shoat in they're pen. Gradually increase the size as the pups mature, building confidence and "firing them up". Don't do too much, as it can bore them and they can loose interest. When they've mastered the pen idea, move to the woods as soon as possible, but make it easy and fun(i.e. so they don't have to work too hard the first couple times). Once they're hunting age(anywhere from 6-12mnths) make sure to back them off on a rough hog, which will teach a dog some respect, hopefully avoiding silly mistakes that could cripple/kill your dog. I like a big rough hog about 200#'s (filed down teeth) that will beat one up a little... sort of teaching a dog how to avoid the "dangerous end" of a hog. I've seen way too many good dogs ruined by skipping this last step... one or two sessions of this will really save some headaches
Just my way, but I'm sure there's a million better ways to do the same thing