It's possible that as an author, Tolkien was subsuming the term 'years' under the subjective experience of each race and generation. While he was scrupulous about creating a consistent world in all its details (fortunately, since our discussions would be pointless otherwise) his descriptions of the fellowship's experience of time in Lorien suggests that time is somewhat fluid in the presence of the Eldar and their magic, and I would think the closer the presence of the two trees got, the more slowly time might flow, so that a year by the light of the trees would indeed be comparable in experience and in matters of the heart and family to a year by the light of the sun and moon. The eldar might be physically and psychologically responding to the turning of gold and silver light, and then the turning of daylight and darkness, rather than to the absolute chronology of minutes and seconds, as measured by an observer outside of middle earth, Eru with a stopwatch, say. Correct me if I'm wrong (my Silmarillion has been packed up in my parents' attic for 15 years or so) but the examples of marriage and childbearing are all from the time before the trees were destroyed and the sun and moon created. The making of the sun and moon may have changed the elves' responses so that the rhythm of their lives speeded up to match the new shorter length of a year. This may have entailed a slowly gathering weakness in them, and so brought about their gradual decline as a people, as they were forced to live day by day and year by year too quickly for them to thrive and increase. Thus, there were no more elven families with 7 sons. This would be yet another tragic consequence of the destruction of the trees.
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