Both breeders who I have been talking to recomend adding in veggies at 4-6 months old. I've heard of people feeding veggies earlier, but I don't see a reason to rush it and upset the GI tract possibly and for no reason.
I would never feed fruits or veggies until at LEAST 4 months old, but I always wait until 6 months. No need to be hasty and cause digestive upset, right? And when I do, I always start with vegetables, introducing one new thing at a time for a two week period before introducing anything else. I usually start with romaine lettuce, then spring mix, then cilantro, then carrots, and then the fruits are last around 7-8 months and then only very sparingly.