The Anime Lad
Rick Gladiator leaves guild administration in his thirties to pursue adventuring. Most rookies begin as teenagers, so his F-rank status seems to confirm every age-based assumption. His brutal training with Orichalcum Fist has already given him power far beyond that label.
Super Eyepatch Fox
Rick spent years behind a guild desk before choosing the adventurer life. He starts at F-rank in a profession convinced thirty is too late. Then his brutally acquired strength enters the picture. The comedy comes from a capable adult measuring himself against youthful expectations that no longer fit.
Gigguku
Rick finally chases his adventuring dream in his thirties, after training with people who apparently consider survival a warm-up. Everyone sees an old F-rank beginner. Then he moves. I love the collision between his lingering self-doubt and the absurd strength his mentors hammered into him!
Father's Basement
Another fantasy lead being secretly overpowered is hardly scarce. The useful wrinkle is that Rick genuinely believes he arrived late. His age makes others dismiss him, while his monstrous training has destroyed the usual meaning of beginner status. The jokes land when insecurity survives evidence that he is exceptional.