A measure of the age of a young Elf, translated “growth-year” (NM/84, 120). It seems to be derived from a combination of √OL “growth” and √MEN “go, proceed”, so perhaps originally “*growth-process”. It may be distinguished from a coimen “life-year” as a measure of the age after maturity (quantolië), a longer span equal to one yén which is an Elvish long-year (144 sun-years). These measurements are complicated by the fact that (a) the age of maturity is different for Elvish men and woman and (b) Tolkien kept changing his mind about how quickly Elves aged during childhood.

As originally described by Tolkien in notes from 1959 (NM/83-92), Elvish children grow at roughly 1/12th the rate of human children and one olmen is 12 sun-years (löar). Elvish women reach maturity after 18 olmendi (216 sun-years = 1.5 yéni) and Elvish men at 24 olmendi (244 sun-years = 2 yéni). Physiologically, a 216 year-old Elvish woman was the equivalent of an 18 year-old human woman, and a 244 year-old Elvish man was the equivalent of a 24 year-old human man.

Upon reaching maturity Elvish aging slows dramatically, so that after one yén an Elf ages about the same as a human would in one sun-year. Thus a 5 yéni-old (720 year-old) Elvish woman is the physiological equivalent of a 21.5 year-old human woman (18 olmendi ≈ 18 years + 3.5 colmendi ≈ 3.5 years) and a 5 yéni-old (720 year-old) Elvish man is equivalent to a 27 year-old human man (24 olmendi ≈ 24 years + 3 colmendi ≈ 3 years).

In later notes (but still from 1959) Tolkien changed his mind, so that Elves grew in childhood at the same rate as humans and reach maturity after 24 sun-years for both Elvish men and woman (NM/119-120). Thereafter they aged at the slower rate described above, with 1 yén the equivalent of one year of human aging. In notes from 1965, Tolkien changed his mind again, so that Elvish children matured at one-third the rate of Men and reached maturity after 72 years, but still the equivalent of a 24 year-old human (NM/146-148).

Regardless of the actual growth rate in childhood, the term olmen can be used to describe the age of children, whereas coimen is used for age in adulthood. Elves with the same number of coimendi would have the same level maturity and life-experience, but their temporal age could be 1-2 yéni more to include their childhood olmendi. Perhaps the same terms can apply to Men, but in that case might measure only a single sun-year given the shorter life-spans.