I began outdoor training Jackson the first day I brought him home at 9 weeks old. He had only had 1 set of vaccines at that point, not getting his 2nd set until 12 weeks old. I didn't want to mess with the pads because I didn't want him learning to go potty inside was okay, pad or not.
Now, obviously, there was no taking him to dog parks, Petsmarts, places where a lot of stranger dogs have been, etc but he was taken out on leash in our backyard as well as let in my dad's fenced in yard with his 2 dogs. Jackson has been an outdoor dog from the beginning and I could never contain him
With the first month of owning him (got him in Dec. 08) he had experienced snow, rain, mud, grass, outside, etc and he was highly socialized with other dogs (that we knew were updated on shots, etc) and now he's a very well rounded dog and I think I owe it to the early socialization I provided him by not keeping him confined to a house. Also, after he had his 12 week appt. at the vet, she told us it was fine to begin potty training outside.
I think you just need to be aware of common diseases in your area, certain places are more susceptible to lymes, some parvo, etc. So just beware of that!
This was Jackson at probably around 15 weeks old or so....