This partly hereditary problem can occur at any age, although the risk increases with older dogs. It usually results from a number of factors of genetic origin (such as a deep chest, lassitude of the muscles that sustain the stomach in the abdomen, excitable temperament and voraciousness), combined with a diet excessively rich in carbohydrates and insufficient time allowed between meals and exercise.
The stomach bloats following excessive drinking, eating or air allowing, and might twist around itself, strangling the oesophagus and the pylorus and blocking blood circulation. The problem requires immediate intervention by a vet, who will proceed to empty the stomach and eventually perform a gastropexy, a surgical procedure that fixes the stomach to the abdominal walls and should be done within the first eight hours.
Dogs who survive a gastric torsion should undergo surgery, because the risk of further torsions is high. First symptoms consist of an increase in abdominal volume (especially on the left side) with the stomach being hard, fruitless attempts to vomit, restlessness, panting, standing still, walking slowly or lying down. If these symptoms present themselves, you should rush to the vet because two hours are enough for the animal to die in agony.
Bloat is not always necessarily followed by torsion; the dog can have his abdomen dilated and hard but no other symptoms. This condition tends to normalize by itself, but the animal will be a lot more prone to having a gastric torsion, thus a gastropexy is generally recommended anyway.
(excerpts from my book "Cuidar do Cão da Serra da Estrela / Rearing the Estrela Mountain Dog")
Images from here.