Elves are functionally immortal. That is: if they don't get shanked by a rusty goblin knife, burned by dragonfire, torn limb from limb by a troll or meet some other sticky end, they have the capacity to live more or less indefinitely (at least until the world itself turns to cinder).
Like the natural world to which they are intrinsically linked through their fey blood, elves go through cycles of death and rebirth. An eleven life is not a straight line from birth to death, but rather a cyclical process, mirroring the seasons.
First of all, there are no elf children. Fey cannot give birth and thus stole human babies (replacing them with changelings) to increase or sustain their numbers. This was purely for show: although granted longevity and fey aspects, the stolen humans still grew, aged and died. They were good for parties and filling the seats at midsummer feasts, but they faded. So eventually the elves stopped taking them. Instead they contrived spells and charms to copulate with humans. That kinda backfired, too. Half-elves became as estranged children, still mortal and incapable of fitting in elven society. Sooner or later they all left for human lands, though they found them less welcoming than they hoped.
The absolute number of elves is inevitably falling and some say that when the last elf gives up their last breath, so will the last tree, the last flower, the last briar bush at the end of the green. Elves know this, but they also know there's no stopping it.
Since there are no elf babies, the "beginning" of an elf's life can be arbitrarily said to be their spring phase. Spring Elves craw out of their cocoons fully formed, with peachy pink flesh and dewy hair. Having been revitalised and freed from the darkness of their previous cycle they tend to be cheery and spry. Like children with no concept of death they are still drunk on their renewal, fully confident in their immortality, foolish and prone to take risks. Other elves regard them with some measure of contempt and a sliver of worry (perhaps bordering on fear), knowing that a Spring Elf is the greatest threat to both themselves and their species.
About a human lifetime later, the peachy skin grows darker and tanned, their hair stiffer and radiant like burnished gold, radiant and oppressive as the midday sun. Summer Elves are more parochial and conservative, their physical change makes them aware of the passing of time and the dual nature of their immortality. They become haughty and tyrannical in their ways, pursuing more elaborate and Machiavellian ambitions. This is the phase of their life when they make plans for hundreds of years down the road, when they seek to imprint themselves on the world at the height of their power, when they seek to rule the passing world and make it stand in awe at their glory.
Then another lifetime later, their hair begins to grow auburn or red, their skin pales to a parchment yellow. Autumn Elves begin to feel their power wane and while they may still lash out and fight with the relentlessness of autumn rain, they begin to withdraw and pursue more melancholy goals. This is the time when the greatest elven art is produced, when they spend decades on a single stanza of a single poem, secluded away in some thorny court or wandering the forests composing symphonies of a thousand birdsongs. They leave behind their plans to shape the world for another lifetime and instead begin to observe it, mourn it, and embellish it in little ways.
As the last phase comes to a close, their hair loses its colour becoming first silvery and then white, while their skin pales to a maggoty white. Winter Elves are cruel and bitter, with the temperament of a petulant child or resentful old man. They withdraw further into themselves, capable of little but hatred for other people (including their kin), a revulsion grown from the fears and insecurities of their own degrading state. As their bodies turn frail and gaunt, so do their minds lose their edge. The further they slip the more deranged and spiteful they become, reacting to the outside world like a hissing night creature caught in the lamplight. During the very last months of their winter, they begin to involuntarily secrete a milky sap. At which point they crawl up in some dark hole tearing up their old diaries and poems, spellbooks and love letters (or dry leaves and their own clothes, if paper is unavailable), mixing the paper with the sap and building a cocoon.
While Spring Elves are the most danger-prone, Winter Elves in their cocooning phase are both at their most dangerous and at their most vulnerable. Some of their kin might attempt to guard them, if they know where they holed up, but it's wise to keep your distance. Some mortals foolish enough have been known to attempt to "liberate" a powerful elf's spellbook before it was torn to bits by its owner. They did not count on the boundless spiteful savagery of a decrepit fae.
Once cocooned, it takes a few days for the elf to dissolve into opalescent goop, which then congeals and forms a new Spring Elf. These retain most of their memories and personalities, but they are invariably changed to some degree from their previous incarnation, and often lose a a significant part of their self in the final weeks of their madness.
The entire cycle lasts somewhere between 240 and 480 years, depending on various circumstances. If you wanna play a seasonal elf, pick your incarnation's current season or roll 4d100, with each season lasting a 100 years. The ageing process is slow enough that it probably won't enter play, but there are many spells and curses that advance ageing, as does spending time outside the elflands or being subject to harm. If you're an elf that lives the adventuring life, add 1d10 years to your age every time you drop below half your HP or reach 0 HP.
There's no mechanical distinction between the various phases, although I might write them up at some point.