hmm well if you want to look older, you could try less colorful clothes, like greys and whites and blacks, but still in cute cuts. you could also buy some glasses lol and wear your hair up. and maybe some heals. These might seem old ladyish, but thats kinda what your going for right?
or on the other hand, maybe you should just wear what you want and not worry about it, if youve got the skills, youve got the skills and people should take you seriously by your actions.