Beechbone was burned, apparently fatally, during the assault of the Ents on Isengard. We're not told how old he was at this time; the timeline above assumes that he was extremely ancient, as were many of the Ents, though we do know that others of the Ents came into being later than this. In principle, Beechbone may have been born (or perhaps grown) as late as the Second Age.

Presumably Beechbone's original name was Entish, with an Elvish contraction that was the origin of the name 'Beechbone'. Neither of those names is recorded, but the Elvish form presumably incorporated the element brethil, 'beech'.