Definitely jerks will find reasons to be jerks, yep. For one it's gender, another sexual orientation, another an age-gap, another spirituality, another diet, another medical treatment choices, another religion, another youth (or perceived youth).
I fully agree with the tips re: body language, speech, etc. i.e., The issue isn't that you look young; the issue is that there are some rude, ignorant jerks in your environments. The latter is what needs to be navigated, and the tips given here are great.
I've had a similar experience of having much more success working (in my case) via email. By the time they meet me, they think I'm older, wise, intelligent, organized, focused, etc. It's much easier doing it in this order than getting past all the barriers they create the moment they see young, female, etc. I guess that technique is called: surprise them with our faces ;)
Your competence and EQ are what will sort things out in the long run, but as others also said, it will take time for some to get past their own barriers to see those. That's one of those crappy things, for sure. If you are patient with their barriers, most will come clear.