The condition is very common except among people of European ancestry. For the lactose intolerant, dairy products containing significant amounts of lactose cause diarrhea, abdominal cramps, and nausea. Yogurt is easier to digest because enzymes from the yogurt bacteria break down some of the lactose during the fermentation process. There's also preliminary evidence that once the yogurt's eaten, the bacteria may continue to help break down lactose in the intestinal tract. As for other claims about yogurt, questions remain.
Few today claim eating yogurt ensures longevity.
The Bulgarians, for example, practiced a number of healthy habits besides eating yogurt, so it's impossible to say their passion for curds and whey guaranteed their old age. But other health claims circulate, fueled primarily by the yogurt industry and sometimes encouraged by scientific studies of yogurt. I am seeing enough research that I would no longer 'pooh-pooh' these things as silly. But I think we need more research before we can be sure. It's certainly nutritious, it can't hurt you, and it might even help.