Perhaps it's unavoidable (Score 1)

I've had a similar experience being 21. My skill set doesn't coincide with my experience, so I would appear unqualified on paper. At my last job I was in many ways taken advantage of because of my age. There were many assumptions made about me, but I really can't blame them for this. A vast majority of people our age, as well as people that would descriminate based on our age, are/were more interested in getting fantastically entoxicated and getting some than taking their careers or anything else seriously. That's not to condemn their behaviour, more to state that if I were given the task RIGHT NOW of hiring the best people I could for the job I more than likely would not start looking at the 20 year old bracket first.
It's certainly unfortunate though when people take this too far. You have to actively take care of yourself. I don't think there's a way around this. All you can do is be a stand up guy and if you feel you're being mistreated bring it to their attention in a calm way. Anyone willing to listen will realize that they are in the wrong; and anyone else just can't be helped.