Shinmai Ossan Bouken-sha, Saikyou Party ni Shinu Hodo Kitaerarete Muteki ni Naru.

Summary

"Why should it matter how old you are if you want to start something new?"

The strongest guy around, who also happens to be middle-aged, strives to be an adventurer! Everyone seems to think you should pursue your dreams while you're still young, but there's one man who just can't give up. In a world where most people who become adventurers do so in their teens, a guild employee in his thirties, Rick Gladiator, seeks to become an adventurer instead. After receiving unimaginably brutal training from the legendary party renowned as the strongest on the continent, Orichalcum Fist, he ends up with combat abilities worthy of S-rank status despite starting out as an F-rank rookie!

(Source: Crunchyroll)