But if he's NOT authority....
Well, I dunno if it's very Christianly, but if it were me, I would just spew. "Look man, you don't see ME going to YOU and seeing what you're doing in YOUR life, do you? So I suggest you mind your business and leave me be. I'm not here to please you or any other person, and I don't have to answer to anyone but God and my parents. YOU don't look like either one of them. So just...MOVE ALONG and find something better to do with your time."
But that's just me. And that's probably a bit nicer than I would really say it. I'm probably more likely to be like, "What the heck is it NOW, huh? Am I not WALKING properly either? GO AWAY." Hmm...that's too nice too. Oh blah. I dunno heh. But I wouldn't want to be nice about it.