Only on the front line at the very end. Before that they had been sent to backfill anti-aircraft and rear area soldiers to free up men for the front lines. (That’s how the last Roman Catholic Pope got his war experience with an anti-aircraft unit). One source I saw gave January 1943 for the HJ replacing the regulars but that was when they started working as Flak helpers, not when they replaced the regulars. I recall one Luftwaffe guy assigned to a searchlight battery who didn’t get sent to the front until late 1944.
My mother’s cousin was one of those kids. He was fifteen. Got captured by Americans in the last week. When they found out how old he was, they kicked him in the ass and told him to go home.