Pokemon death has already been brought up with the presence of Pokemon Tower, in which trainers bring their deceased pokemon to bury them and such.
Pokemon do seem to age, as shown with Surfing Pikachu, who grew up with his trainer (in the anime), as well as Tracy's Scyther, who is said to be older than the rest and is, thus, a bit less physically fit than the others.
As far as a clear-cut maximum age limit is concerned, I don't think there is one. There are a good deal of pokemon -- such as Ninetales (Kitsune) and Espeon (Bakeneko / Nekomata) -- that, by design, would require a relatively high lifespan in order to exist in their current forms. If I recall, Vulpix alone would have to have existed for a good deal of time just to get its current 5 tails, and yet they are simply born as they are. I must also mention that I have yet to see a bug-type pokemon, aside from Tracy's Scyther, that demonstrated any effects due to age. If they had such short lifespans, then the variance due to age would become obvious with Beedrill swarms, as a Beedrill even a few weeks older than another would have a noticeable difference. Contrary to this, however, it can also be said that their lifespans are *so* short that they don't even survive long enough to appear old.
This is countered with the continued survival of the Butterfree that Ash happened to release early on in the series, as it does manage to appear later on in the series.
I, personally, believe that a pokemon can live indefinitely provided it does not cease being active. Even with great age, a pokemon that continues to be active would maintain its strength and power. Once a pokemon stops doing this -- such as if its trainer dies and it becomes disheartened, or pokemon that are kept inside the house (such as Skitty) -- it becomes more and more vulnerable to the effects of age and may even perish.
This would also explain just how pokemon battles came about. In order to maintain the lifespan of their pokemon, it would be understandable to pit them in combat against each other with the knowledge that doing so would keep them alive for a longer period of time (barring fatal injuries). From there, it could easily evolve into pokemon battling growing into a recreational activity or even a professional one.
But I'm reading a lot into it.