Longevity can vary- while a Freebirth's natural lifespan is about that of an exceptionally fit human, genetic pokemorphs (ie: First and Second Gens) have a degree of random to their lifespans determined by the morphing process- some live a little longer than humans and Freebirths, some a little shorter. This is entirely because aging has everything to do with the state of the cells in a living body. For example, even though Fault was starting to reach an older age, as far as pokemon go, he basically returned to the prime of life when morphed. However, the sheer amount of variables involved in the procedure can mistakenly produce a much younger or older-appearing morph, particularly in the more unstable First Generation. In addition, further aging can be either slowed or accelerated by the process. Some morphs' cells are less stable, causing them to age much more quickly, while others will stay youthful for quite some time. The base pokemon species has no bearing on lifespan, rather, the variance comes from the quirks in the Change process and Agent Armageddon. However, the extension or reduction is never more than perhaps a decade or two in either direction.
It should also be noted that, as the MUSH runs on a timeline parallel to our own, aging according to the progression of months and years is required for all characters. The staff may give permission to bend this rule due to extreme OOC circumstances, but one must speak with a staff member to obtain permission.