Styling advice wanted

Give a man in need, a style advice! Let's talk about mens purse for a bit! More exactly belt bags, not the formal, office ones, but something more casual/sport, for weekend walks. I like how this guy wear it, very effortless and manly! It gives a teenager vibe, still looks confident enough. I think I'm gonna buy one, but I'm not yet sure how it would look on me. How should I wear it to avoid looking like a high-school boy? :P