[Carannillion]

Perhaps, if it is so that Legolas is older than Elrond, the reason for the seemingly 'youthful rashness' of Legolas is due to a life in the woods, as a fighter/hunter, going along with his childhood friends, while Elrond has seen quite a lot. Elrond is perhaps more a political type or leader than Legolas; always striving to learn more about everything and employing it to his - and others' - use in the battle against Sauron (a heavy burden). Legolas is specialized in being a royal heir which will not take the power until his father dies (which is probably only going to happen in a war) and so he simply protects his land, turning all his efforts in this direction. He does have som knowledge of ancient lore and such, but nothing compare to Elrond.