The information provided by
FemElf comes from a text called
Laws And Customs Among The Eldar (short version of the title) which was published in
Morgoth's Ring. It's a popular citation with respect to this matter, but in any case it's not the only idea that JRRT ever wrote (not that anyone said it was). For two more examples...
Quote:
'They' are the Númenóreans: 'Thus (as the Eldar) they grew at much the same rate as other Men, but when they had achieved 'full growth' then they aged, or 'wore out', very much more slowly.'
Note 1, The Line of Elros, Unfinished Tales
And with my emphasis, from the author's notes to 'neter, kanat, enek', note 1:
Quote:
'C.E. ? netthi. C.E. tth > Q., T. tt; S. þþ > þ. nette meant 'girl approaching the adult' (in her 'teens': the growth of Elvish children after birth was little if at all slower than that of the children of Men). The Common Eldarin stem (wen-ed) wendé 'maiden' applied to all stages up to the fully adult (until marriage).'
JRRT, from Vinyar Tengwar 47, texts generally dated 1967-70
I note the date on this last text -- thus this idea appears to be later than that found in
Laws And Customs.