I have this problem....

I have a problem where I am 21 so people think I am extremely naive. Then my face and height looks 15 and everyone thinks I am 15 until I say my age and they don't believe me. 
So I really don't get taken seriously half the time. I ,also, sound either autistic or like a toddler, so some people think I am autistic and I get treated as such. 
Then there is the other problem where all my life people think I am always lying. I was really just being an extremely sarcastic, asshole, bitch and they took it too seriously. Because their stupidity and such just really pisses me off. So yeah..... 
I was hoping this new job I took on would help that. Any other suggestions??