No, you are NOT ARROGANT, but some of us who have poked at you are.
You are just..........ignorant's a strong word..........UNINFORMED. We all weren't born "knowing everything" about drum corps, and you won't learn it overnight. Some of what you are talking about, you learn by doing and can't sit back and analyze it on the chalkboard in class. And, since it appears that you won't have a chance to march in DCI, if there are any all age corps in your area I suggest you perhaps check them out for membership in the future.
P.S. Under certain circumstances (when is the age cutoff again?) you can march at age 22, so you should look into that as well.