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

It is common knowledge that it is best to become an adventurer while you are young. This is because the "magical power" that is the basis of an adventurer's strength can only be developed if the adventurer trains while young. But this newbie adventurer, Rick Gladiator, who only became an adventurer at the age of 30, had the fighting strength of an S-rank adventurer, the highest rank. People thought that the "Mere Old Man" would not make a good adventurer! But in fact, Rick has lived an unimaginable daily life with the members of the legendary "Orichalcone Fist," a party of the continent's most powerful adventurers. From dragons to vampires, Rick has been trained by literally "monster" level masters, and with his fighting skills, he is able to defeat the elite adventurers who underestimate him.

(Source: HJ Bunko, translated)