I'd do it early. The only caveat I'll give you is to make sure if you're socializing with other dogs, I strongly encourage you to do that in the right setting. I don't like taking them to places where there's a lot of other dog traffic, especially dogs that you do not know. So your big chain pet stores that let you bring pets in there—I’m not a big fan of doing that with puppies. Dog parks, groomers, boarding facilities, I will try to find any other alternative when they're puppies. When they're not fully vaccinated, I try to stay away from those kinds of activities for obvious reasons. You don't know what those dogs have or where they've been or what symptoms they may have.
Now if you have friends or family members or perhaps even have other dogs yourself, and they're healthy, and you know that, knock it out. Let them socialize. Let them be a dog.
I currently have a four month old puppy right now and I have a one year old and a seven year old dog. They are best of buds. They do everything together and that's fantastic. The little four-month-old is learning so many things from other dogs, I'm not even having to show him anything. He just follows what the others do and he's learning how to be a dog. That's okay. But of course, I know that those other two are perfectly healthy and vaccinated. In that kind of situation, do it early and often. If not, hold back, and then before you start doing trainers and other things, I would wait until they're fully vaccinated, which in most dogs is closer to, say, four months.

Depends on what kind of obedience training you're talking about, first off. So I will tell you that obedience training that you're going to do at home? As soon as possible and, again, in moderation. You know, with a six week old puppy, you don't put them through bootcamp. Let them be a puppy. If you're going to give them a treat, why not have him on a tabletop like this, hold the treat up high so they're looking at you, and then say, "Sit. Sit."" And watch their rear end drop to the ground. If they're not, hold it up higher. Go above their head so they're going to follow that treat and then they're going to sit. It's a natural response. So that kind of training, heck yeah. I'd do it from day one.
Other small training, like every time I feed my dogs, I put the food down. I make them sit or at least stay and I'll put the food down. I do not let them attack the food bowl. It's by design. I want them to know that they’re going to do what I'm asking them to do. It doesn't hurt. I'm not being mean in any way, shape, or form. But they do kind of learn that they have to do things my way and not their way all the time. And you would be amazed at how many things that spills over to. I mean, keep in mind, I'm a veterinarian. So I see dogs walk into exam rooms all the time. A dog that is well-disciplined even if they don't know 50 tricks, that's fine. But even if they know discipline, they know to sit, they know to stay, they know to heel, those kinds of things—that is such a big advantage for us and for the owners.
It just makes for better dogs when they have discipline like that. So I implore you that even if it's small things, train when you can in moderation. Don't do it for 30 minutes a day. Do it for three or four minutes a day and stop. They’re still puppies. Make it fun. When it's not fun and they're losing interest, stop. Do it again the next day and you'd be much better off for it.

I'm going to say yes and I'm saying this while I tiptoe walking on eggshells because I don't want to rub anybody the wrong way. Dogs need discipline no different than a child needs discipline, so yes. If I stuck with a short answer, I would simply say yes, but I feel like I have to explain.
If you catch them in the act of doing something undesirable, such as messing in your house, chewing on your favorite shoes, fighting with another animal, trying to bite you too hard, or anything like that, it requires correction in moderation. It doesn't mean you have to go smacking them around. A lot of times it's just a loud noise. And most dogs when they hear that and they stop, that's correcting in itself for probably 80% of puppies. Have I at times tapped mine on the nose if they're really not getting a message, if they're not responding to an auditory stimulus or something like that? Yes, I have. Is it okay? I think it is in moderation.
again I am not advocating going around smacking your dog. But they do need correction. They need discipline and they do better when they know what's expected of them exactly like a child would be. If caught in the act, yes. If you were trying to correct them for something that happened maybe while you were at work, it might've happened hours ago, nah, you're wasting your time. And I have learned the hard way, do not even bother because they will have no idea what you're correcting them for. And then it just becomes a thing where, oh my goodness, now they're fearful of you because you're just this big old guy that just comes in and starts spanking them for no known reason. So if you catch them in the act, yes, punish in moderation, otherwise let it go. Deal with it the next time you see that behavior happen.

Exercise is probably the best bet. Think about a puppy. They play hard for an hour or two just like a child. You look at them and you think, ""Oh my God, where did all this energy come from?"" But then right after that it's lights out and they're going to be asleep for an hour or two.
So with that being said, the best bet with a puppy? Play with him. Give him an outlet for that energy. Take him for long walks. Play fetch if they're old enough to do that yet. Anything that's going to use energy, use their mind, that's that's what they're bred to do. You know, they're pack animals. They’re normally roaming around with other dogs all day long or sleeping. That's not going to change just because you got them and brought them into your home. So they still need those basic essentials. 
Let them be a dog. Let them explore. Take them for walks, do those things to burn off some of that energy. And I will tell you one other thing, too, as far as training goes, the best time to train them is right after you've done that so their energy level is not so high. They’re in a better mental state where they're more relaxed and much more amenable to learning new things.

Hopefully, it's not true aggression that this question would be geared towards. I have seen puppies that are aggressive at a very early age and that's a little bit disturbing. More times than not, it's just playing. They're trying to find themselves in the pack. They're trying to find out who's the alpha and who’s not. What can they get away with? They're learning how to interact with dogs. And if you've ever watched a pack of dogs, wolves, any canine species, that's how they are. It's what they do. So the playfulness/aggressiveness is okay for them to do those things in moderation, as long as it's in play.
If it becomes growling, snarling, pulling, and as if they have to have the last word, that's a problem, and that needs to be addressed straight away. I'm not a trainer so I'm not going to stand here and try to give you this long, drawn out process on how to do it.
But one thing you might want to do is consult with a trainer and see the right methodology on how to break that or steer that energy elsewhere to where it becomes productive and not leading down a not so good road.