What's the name of the manga where the main character is a Mage and his name is Luck?
After spending years fighting demons, he becomes younger from absorbing the Demon King. Mana is still a fairly new thing.
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I believe you are looking for I Became a Legend a Decade Later After Telling my Comrades to Leave Everything to Me and Retreat First.
The main character is a Mage named Luck. The first chapter start with him, and two other heroes, having just taken the demon lord's arm, causing him to withdraw. Soon afterwards, the demon army is upon them. Luck has the others head back while he holds them off, since they have families, unlike him. He then fights the demon army, eventually learning draining touch from fighting one of the demons. He then fights more demons until the demon lord reappears. He then beats the demon lord using the draining touch, which makes him younger.