It is as typhoonpilot says--18 to 21 are overlap years for cadet to senior. I stayed in the cadet ranks until 21 (I was slow).
Yeah, some squadrons (or groups, or wings

) can be really bad. There are some people in the CAP who are there only to play soldier and lord over their fiefdoms. They make things tough for the good guys. I admit that's what I finally got tired of after 31 years. That's why it's so necessary to shop around for a unit one can fit in with.
Yes you will wear a nomex flightsuit (if you meet USAF height/weight/grooming standards) or CAP jumpsuit (for the fat/bearded guys) when you fly, even in a C-172. It's a USAF requirement. If you think that's silly or you don't like it, you don't have to fly.
The only real operational requirement if you want to fly is to pass your initial and recurrent checkrides and stay proficient. On the PME side everyone must go through "Level 1" training which covers the basics about the organization, regulations, insurance, liabilties, and cadet protection. Also, most people pick a technical specialty track that they're interested in and earn at least a technician rating in it, but that is voluntary.
All training after Level 1 is voluntary. But if you really get into it, there's various weekend technical, management, and leadership schools available at Levels 2 thru 5, including USAF ECI and residence courses. If you aspire to a command or staff slot above the squadron level you will go through some or all of those additional levels of training.
I went through the whole thing, completing National Staff College and Level 5. Held master ratings in cadet program management and communications, and a technician rating in aerospace education, along with a command pilot rating and a grade of CAP LTC. Highest staff jobs I held were deputy commander for cadets at the squadron level, IG at the group level, and air operations officer during SAR missions. I had a blast.