Although I was probably aware of it earlier, I first realised the formality of my speech when I was 17. My mother managed a pub, and one of her regulars commented to her that "your lad talks right posh dunnee?". There's a lot of unusual slang in the local dialect, which I only ever use in a self-derisive manner.
I have made an effort to use contractions in my speech, but I still find it very difficult to 'fit in' with people with a heavy dialect. Despite this, I find people are generally accepting of the way I am. In my view, anyone who judges me on my speech patterns is not worth my attention anyway.