IMHO, a race that can live "for centuries" has a longevity that should not appear in the experience of a normal PC. The rules designers purposefully limited PCs to species whose life experiences and physiology would be comprehensible and familiar to what human beings experience in the real world (allowing for deviations due to the existence of magic and the supernatural, of course). Taking humans a baseline, the further you deviate from that baseline, the less ability a player would have to identify with how such a creature's life would be experienced. It's why only 1-HD creatures are normally allowed to be PC races, but things like angels, dragons, and giants (though humanoid) are not, house rules excepted.
But my complaint is this: longevity is not, in the great scheme of things, uber-powerful like SR or spell-like abilities are, but from the real world perspective, it is (or should be) regarded in the game as a tremendous benefit. The ability not to die from old age is vastly underestimated by gamers and it's only because so few PCs and campaigns even last that long. PCs retire and become NPCs when they have outlived their usefulness as a toy (for lack of a better word) of their owner, which can happen far sooner than circumstances when the age of the PC might be important. This only occurs in one of three ways: the GM/DM inserts a deliberate fast-forwarding of the campaign for some reason (the party retires from adventuring for a few years, and nothing important happens during that time), or part of the group visits a plane of slow time or timelessness (meanwhile friends and allies back home age normally), or a player deliberately designs a PC to be aged for his or her race. None of these are very common circumstances, but this does not mean longevity is a thing to be taken lightly.
Perhaps it is just my preference, but I tend to be concerned more with personality, character development, and the story, over the "kill monsters, take their stuff, get rich and powerful" motivation that characterizes the hack-n-slash approach to gaming. But even a character who wastes away the flower of youth raiding dungeons and furthering the cause of natural selection amongst the degenerate species of the world will make more money, and accumulate more power, if he or she has more time to do it. If he's an elf, he will have a few solid centuries to spend liberating treasure from its former owners, than if the PC were a human.
Most players cannot conceive, realistically, of being able to live for a thousand years. Most of us know people who have lived into their 60s, 70s, or even 80s and beyond--if we're lucky, we might even meet a centenarian. But who can imagine having lived a life that might have included living through the Great Depression, fighting in the Napoleonic Wars, witnessing the Spanish Inquisition, acquiring a freshly printed Gutenberg bible, or escaping the Black Death?
Writers of contempory fiction try to encapsulate the experience of having an unnaturally long life through such tropes as being a vampire, an angel or demon, or some other immortal creature. The believability of such stories set in the modern world hinge on the rarity of such beings. In a fantasy world where one of the dominant races a lifespan of centuries presents problems that GMs/DM have been asked to address or have attempted to explain away on their own: If elves live so long, why aren't we hip-deep in them--why haven't they dominated the world through mass population? if elves live so long, why haven't they dominated the world through technological/technomagical development? Answers have varied across the board. Or, the GM/DM just doesn't care, and leaves the question unaddressed (which is OK too).
My preferred approach is to discount the longevities given in the standard rules entirely. In my campaigns I would rather define a span of about 400 years (still a very long time) as the absolute maximum age permitted any PC race, and that only for the most ancient of elves, and excluding beings which have used magic or undeath to prolong their life. Perhaps the standard longevities applied in ages past, much like the patriarchs in the Bible were said to have lifespans of 700-900 years before Divine Decree shortened human lifespans to 120 years (see Genesis 6) and again to about 70-80 years in Moses' time (Psalm 90). So elves might have lived seven or more centuries in ancient times, but these days they are very fortunate to see their four hundredth birthday. The longevities of the other PC race would have to be proportionately shorter, but all still living longer than men.
