I got hit hard (...bad choice of words...) by 9/11, but not in the usual way. I got to Sunday School (parents are Christian Scientists) after the fact, and they told us death didn't exist and my 12-year-old-new-to-thinking brain flipped out. I emotionally left the church, but had to go to Sunday School once a week for the next 8 years of my life. Thus there has already been a connection in my mind between control (and choice) and 20. Thus, for me, when you get formally recognized control over one part of your life, it somehow opens you up to make other conscious choices about control in your life.

I also think, again at least for me, that fact that I am dismissed based on age less is due to exactly what you were saying about controlling how people perceive you. I feel more adult, so I behave more adult. (I let myself get away with a lot of teenager crap, have stopped letting it slip through now that I consider myself more adult.)