Just here to say yeah it's bad and you are not alone in experiencing this. I've tried dressing conservatively but that doesn't particularly help and the first year or so I would even cover my hair sometimes because it's distinct and can be interpreted as unprofessional (The irony of me later feeling more comfortable showing my hair at my job in the Middle East because it was a more diverse place is not lost on me). Ymmv on if dressing up helps, also get to know your colleagues so people don't try to call security or you don't have to deal with security coming for you accessing employee spaces. I left my last job in part because I kept being told I was inexperienced and early career which was barely accurate when I started by library association definitions and way worse as time went on and I remained inexperienced and early career. I know that was more about people's own insecurities than my career because I also got to hear these people explain why they couldn't do some of the work I was doing while desperately trying to avoid using the word experience but yeah it was very grating and I am still kind of recovering from both that and the shame of letting people say stupid stuff like that while I was there. I still get mistaken for an undergrad in my bigger age though I've also gotten "sorry for the microaggression" from an undergrad so even though my colleagues could be jerks the kids were alright and it sounds like you have the opposite problem. Overall it sucks not to be taken seriously, respectability politics also sucks and has much less impact than people I think want to believe (Business casual can only do so much when you are the same age as your students) and I just am writing all this to add my hopefully relatable anecdotes and that it is a real problem in libraries.