Climbing is largely muscular work. Descending is where joints absorb impact repeatedly, and a sustained mountain descent does it thousands of times in an hour. A dog rated 4 out of 5 on energy will do it enthusiastically at eight months old and show no sign of complaint that day, which is exactly why owners keep doing it.
Denver makes the temptation unusually available. The city owns thousands of acres of mountain parks west of town, Jefferson County's open space network sits on the doorstep, and the local default for a fit dog is a foothills trail at the weekend. None of that is bad for an adult Welsh Springer — it is close to ideal. The adjustment worth making is temporal: keep the sustained descents out of the first twelve to eighteen months, while growth plates are still closing, and build mileage on flat ground instead.
Denver happens to be well equipped for exactly that. The Highline Canal Trail runs about seventy-one miles across the metro and, because it follows a canal, is close to level for its entire length. The Cherry Creek and Platte River paths add more. That gives you a way to satisfy a high-drive puppy's need to move without asking a growing joint to brake down a thousand vertical feet.