Here's my fuller argument...
Morgoth's Ring provides 50 years, for some 100 years (as just noted), and while this has proved to be popular on the web, it's not the only idea Tolkien had in my opinion, nor even the latest idea he had. Other examples include:
'They' are the Númenóreans:
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'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
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Author's notes to NKE ('neter, kanat, enek'):
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Note 1: '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
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This text is quite a bit later than Laws And Customs Among the Eldar (Morgoth's Ring), which generally dates to the late 1950s early 1960s.
Anyway these two texts appear [to me] to say that the Eldar grew at about the same rate as Men.
But even this is not the complete story, as there is another indication, from other texts published in Morgoth's Ring (still later 1950s however), that the Eldar, at least early in the history of Middle-earth and in Aman, matured much slower than 50 to 100 years -- and actually the notion seems to be that it took roughly 3,000 years for an Elf-child to become an adult! and possibly (based on a further text written at this time called Athrabeth Finrod Ah Andreth) in Middle-earth (not in Aman) this rate dwindled*, meaning that it took less time to grow to be an adult in Middle-earth as time passed, suggesting Elven children reached maturity faster and faster until at least Finrod's conversation with Andreth.
This idea is wound up with possible drastic changes in the internal chronology, as well as the notion of 144 Sun Years being equal to 1 Valian Year (not roughly 10 Sun Years being equal to 1 Valian Year, as formerly).
It might be 'remotely' possible that all these ideas could be parts of the same idea, the notion being: a rate of growth that dwindled in Middle-earth as time passed, if the rate rather drastically altered in Middle-earth over time that is: thus 3,000 or so years very early on (and in Aman), but after time the Elves matured faster and faster, and at some point this rate reduced to 50 or 100 years, perhaps at the time of Finrod's conversation with Andreth* -- and even later, the Eldar ultimately grew at much the same rate as Men.
I think that's much less likely than Tolkien simply changing his mind however, as 3,000-ish to about the same rate as Men is quite the change internally! It was probably easier to keep track of chronology if Elves simply matured at the same rate as Men, but that's a total guess as to why Tolkien seems to have later headed that way.
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*'This I can well believe,' said Finrod: 'That your bodies suffer in some measure the malice of Melkor. For you live in Arda Marred, as do we, and all the matter of Arda is tainted by him, before ye or we came forth and drew our hroar and their sustenance therefrom: all save only Aman before he came there. For know it is not otherwise with the Quendi themselves: their health and stature is diminished. Already those of us who dwell in Middle-earth, and even we who have returned to it, find that the change* [*the word change was an emendation to the typescript B (only);
the manuscript has growth -- footnote by CJRT] of their bodies is swifter than in the beginning. And that, I judge, must forebode that they will prove less strong to last than they were designed to be, though this may not be clearly revealed for many long years.'
Athrabeth Finrod ah Andreth (and see Author's note 7 on the Commentary)