Your Welsh terrier might be the perfect size dog; small enough to go places with you, but sturdy enough to keep up. A Welsh terrier usually stops growing around his first birthday and has a life expectancy between 12 and 14 years.
From Puppy to Dog
Fully grown, the Welsh terrier stands about 15 inches at the shoulders and weighs around 20 pounds. As WebMD notes, the smaller the dog, the faster he'll reach adulthood. In the case of your Welsh terrier, that means he'll stop growing around 12 months old. He still may fill out a bit, however. If you're wondering when to switch him to adult food, or if it's safe to buy him a nice jacket for the winter, you'll be safe doing so as a first birthday present. When you switch his diet, be sure to transition him from puppy to adult food over a four-day period, changing the proportions in 25 percent increments -- 100 percent puppy, followed by 75 percent puppy and 25 percent adult, and so on. A gradual change will give his digestive system a chance to get used to his new diet, minimizing the risk of diarrhea or an upset tummy.