I think part of this is the way you act. I was in the post office recently, and I asked for what I wanted in a meek way. I also have a speech impediment and get treated like I am stupid despite having worked fulltime for 30 years, while raising my family
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The cashier was going to take the coins off me, and count them. I wouldn't give them drew myself up to my full height and said i a stern tone, 'I will count them myself, thank you'. She visibly changed and started treating me with more respect.
I know people can't change accents or disabilities, but you could try to act more purposefully and confidently. Sit or stand in a more confident relaxed way. Don't be afraid of eye contact. Speak in a confident tone. If you can, it helps to stay away from dark, baggy, clothes which are a bit shabby. I always wear dark jeans not loose, not tight, and a top/jumper which does not have bobbles. I wear dark low heel shoes. I stay away from runners unless I am going for a long walk. I know we should be free to wear what we want. The fact is, that if one is nearly 50, and is meek and mild, one has to dress reasonably smartly in order to get taken seriously.
A year ago, I had a cold and was walking in town on my lunch break. I may have been walking a bit oddly as I had a headache. Suddenly 6 youths surrounded me, started laughing and imitated me walking. I looked around with head up, shoulders back, looked and smiled at them. They all melted away very quickly.
Another time when I was wearing a coat of 1990s style, two men of 25, looked, said 90s coat and laughed. I walked very quickly after them, laughing and smiling. They ended up running into a shopping centre, no laughing as I power walked behind them.
More recently, a certain type of girl, blonde, 20s, a bit rough round the edges would stare at me for ages if i am in a bank. I usually stare back and raise my eyebrows. They sloewly fade away after.
I think they target people whom they see as weaker, not prepared to stand up to them.
So remember, move purposelly, a confident tone when talking. If someone patronises you, laughing and staring at them while saying thank you, makes them think about their behaviour.