You may be overthinking it a bit...
While my knowledge of biochemistry is limited, as is my medical knowledge, I do know there are several species of marine and reptile life that have amazing regenerative abilities. Star Fish can regrow severed limbs, and the severed limbs can even regrow into other Star Fish! There are several species of Reptiles (Geckos?) That can sever, then regrew their own tails.. (to feed it to predators as it makes a get away!)
Of course, in some cases of regeneration, we are talking about a super natural ability... so who knows? Maybe the Vamps and Ghouls really do live forever? But Shifters do have a lifespan (as noted in their descriptions).... So maybe its both, depending on the source of the regeneration?
If the regeneration ability comes from a biological background, then you're limited to a cellular lifespan?
Which also makes sense, in a perverse way
After all, cellular aging is also part of cellular growth, which in turn is part of cellular reproduction... If your cells never truly aged, then they wouldn't mature, reach the point of cellular division, and eventually die after "X" divisions... They would just remain the same SINGULAR cell they started off as....
OR
Is it just the beer talking??? <After THIS many days in QC/LC its hard to tell>