>>280523321
There's probably a few /m/ protags that also fit, though my knowledge fails me.
Chirico from VOTOMS ws only 18!?

Tetsuya Tsuruge from Great Mazinger comes to mind. Guy has only two things in life; being a combat pro and loving his wife. But I can't find an age listed anywhere and thats not getting into the clusterfuck that was Shin Mazinger Zero where he was a series of clones, all with tweaked ages, memories, fears and traumas to try and find the perfect combination for the ultimate combat pro. I think the youngest being 19