Honestly, I can't imagine anyone worth their proverbial salt overlooking you because you are young - nor because you dress casually. Does anyone dress perfectly to run to the supermarket? I just wear jeans, occasional hoodie, sometimes a nice skirt and shirt, but I don't find the responses to vary unless my attitude is different.
Chav truly is about a behaviour though, something that is immediately obvious and difficult to describe without sounding offensive.....I can't say I've ever presumed someone was chavvy based on their clothing alone.
For example, my taxi driver this afternoon wore one of those chav style Burberry check peaked caps, wore a ton of bling and and a tracksuit top. My eyes kind of noted this but his attitude was sweet and polite so the C word never crossed my mind. Hope that makes sense!
Are you making eye contact and being open/friendly towards people? Sometimes, even with the best will in the world we can unconsciously project vibes that tell people to bugger off. I've done this, it has been a slow and agonizing learning process! Like you, I am currently a bit socially at sea (end of a 20 year LTR, no kiddies, most of my old friends either moved away or super busy with children) so I appreciate where you are coming from. I also work from home so it gets even worse!
Sometimes, we simply need to throw ourselves in headfirst, take the risks and approach people.
And people rarely try to talk to strangers in supermarkets, do they? Nobody has ever approached me, unless their attentions were unwanted 