I don’t think that young is exactly the opperative word, I don’t think Eastwood was that young I think he was in his mid 30’s for Fistfull, Lee Van Cleef was in his 40’s, Bronson was in his 40’s too. Nero was always made up to look older. Eli Wallach was close to 50 for GBU.
I think you need an older star/ actor to pull off a convincing world wise rugged type, but since a lot of American Films are geared towards teens you get a lot younger pretty boy actors and cross-over talaents now.
Viggo has the look, Mickey Rourke could do one I think too.