Why lieutenants faced high Age of Sail mortality

Which officer rank had the highest mortality during Age of Sail combat?

Lieutenant, because lieutenants led boarding parties and performed exposed combat duties around the ship's guns and men.

Their rank placed them at the front of attacks, with midshipmen and junior personnel following behind. Lieutenants also moved through active gun crews and other hazardous parts of a fighting ship. The danger followed from those duties, not simply from how many lieutenants a fleet carried. The same pattern appears in armies, where junior officers and corporals combine direct leadership with exceptional exposure.