In the fnv comic the ncr is shown to have cavalry on horseback. Its propably not shown in the games due to technical limitations and time constraints, the same way the mining equipment in quarry junction is functioning and the miners in sloan actually used it but its never shown in the games
Are they? Body parts replacements do prolong the lifetime to the point of saying that they are ageless, but even that is not exactly true. Biogel can't contain the brains forever - it will become full of byproducts eventually, and the brain itself will start to deteriorate, plus aging doesn't stop. As for the body, it is still fully mortal. Just... Harder to kill.