Elven Life cycle
Owing to their longevity, the Elves had a very different Life cycle than Men. Most of the following information strictly refers only to the Eldar—but much could probably be applied to the Avari as well.
Elves are born about one year from their conception. The day of their conception is celebrated, not the actual birthday itself—since for them, life begins at conception. Their minds develop quicker than their bodies; by their first year, they can speak, walk and even dance, and their quicker onset of mental maturity makes young Elves seem older than they actually are. Physical puberty comes in around their fiftieth to one hundredth year (by age fifty they reach their adult height), and by their first hundred years of life outside the womb all Elves are fully grown.
Elves' bodies develop slower than those of Men from the start. By the age of twenty, they might still appear physically seven years old, whereas Men at the same age are physically mature.
Sexuality, marriage and parenthood
Elves marry freely and for love early in life. Monogamy is practised and adultery is unthinkable; they only marry once (Finwë, first High King of the Noldor, was an exception; he remarried after his first wife died).
Spouses can choose each other long before they are married and be betrothed. The betrothal is subject to parental approval unless the parties are of age and intend to marry soon, at which point the betrothal is announced at a meeting of the two houses, during which the couple exchange rings. The betrothal lasts at least a year, and is revocable by the return of the rings (but is rarely broken). After their formal betrothal, the couple appoints a time for the wedding when at least a year has passed.
Marriage is celebrated at a feast of the two houses. The spouses return their betrothal rings and receive others worn on their index fingers. The bride’s mother gives the groom a jewel to be worn, but the marriage is only achieved with its consummation. Technically, only the words exchanged by the bride and groom (including the speaking of the name of Eru) and the consummation are required for marriage.
The Elves view the sexual act as extremely special and intimate, for it leads to the conception and birth of children. Extra-marital and premarital sex are unthinkable—indeed, the Elves would regard them as contradictions in terms. Because adultery is also unheard of and fidelity between spouses is absolute, spouses can sometimes live separately for extended periods of time. However, a sundering during pregnancy or during the early years of parenthood (caused by war, for example) is so grievous to the couple that they prefer to have children in peaceful times.
Elves have few children, as a rule; (Fëanor and Nerdanel were an exception, since they had seven sons), and there are relatively sizable intervals between each child. They are soon preoccupied with other pleasures; their libido wanes and they focus their interests elsewhere, such as the arts. Nonetheless, they take great delight in the union of love, and they cherish the days of bearing and raising children as the happiest times of their lives.
There seems to only be one known example of extreme marital strife among the Eldar: the case of Eöl and Aredhel, in which the latter actually left the former without his knowing, with the result of Eöl ultimately killing her. This was far from a normal Elven marriage, however.
The Elves, and particularly the Noldor, preoccupy themselves with various things, such as smithwork, sculpture, music, and other arts. Males and females can do almost everything equally; however, the females often specialize in the arts of healing while the men go to war. This is because the Elves believe that taking life interferes with the ability to preserve life. However, Elves do not have rigid gender roles; females can defend themselves at need as well as males, and many males are skilled healers as well, such as Elrond.
Eventually, if they did not die in battle or from some other cause, the High Elves of Middle-earth grew weary of it and desired to go to Valinor, where the Valar originally sheltered their kind. Those who wished to leave for the Undying Lands went by boats provided at the Grey Havens, where Círdan the Shipwright dwelt with his folk.
"Cycles of life" and aging
It is unclear what these cycles exactly are. A logical sense would be that the first cycle is childhood and adolescence, the second is adulthood, and the third is for extremely old Elves; Círdan was the most ancient known Elf on Middle-earth.
Apparently, beards were the only sign of further natural physical aging beyond maturity.
Elves did not age over their 100th year but they aged in a different sense than Men: they became ever more weary of the world and burdened by its sorrows, sometimes appeared to age under great stress. Círdan seemed to be aged himself, since he is described as looking old, save for the stars in his eyes; this may be due to all the sorrows he had seen and lived through since the First Age. Also, the people of Nargothrond had trouble recognizing Gwindor after his time as a prisoner of Morgoth.
Another bearded elf was possibly Tinfang Gelion.
Elves are naturally immortal. In addition to their immortality, Elves are immune to all diseases, and they can recover from wounds which would normally kill a mortal Man. However, Elves can be slain, or die of grief and weariness.
Elves who die or are killed go to the Halls of Mandos in Valinor. After a certain period of time and rest, their spirits (fëar) are incarnated in bodies (hröar) identical to their old ones. They almost never go back to Middle-earth, however. The only Elf known to have done so was Glorfindel, excepting Luthien who returned as a mortal.