The Ossan Newbie Adventurer, Trained to Death by the Most Powerful Party, Became Invincible

Normally, people choose to become adventurers in their teens. At 30 years old, Rick Gladiator bucks the trend by leaving his job as a guild clerk to become an adventurer. He begins as a novice F-rank with the fighting strength of an S-rank. After two years of brutal training with the continent’s strongest party, Orichalcum Fist, Rick will defeat anyone who underestimates him.

Synopsis from Anime News Network