I'm 19, but I'm a moderately advanced knitter, and I always get treated like I don't know the difference between dk weight and bulky weight yarn.
I have the same problem at one of the LYS's in my town. They just outright ignore anyone under 40.
Also, I work at a yarn store, and have had customers judge me based on my age as well. Once a lady came in, walked up to me (I was standing behind the counter), and said, very slowly, "I have a question that you probably can't help me with. I need to talk to an <i>experienced</i> knitter."
The problem with her project, btw, was that she hadn't knit a gauge swatch and couldn't figure out why her sweater was so huge.