I think it does. I am 23, and becoming more extroverted is not on my list of areas of personal growth I would like to pursue, simply for the reason that I am happiest behaving in ways that come naturally to me.
All too often, introversion and shyness become confused. Although I don't want to become more extroverted, I would like to become less shy. I want to be less ambivalent about talking to people that interest me. I don't want to talk more to people who are too hyper to hold a conversation, self-absorbed, etc.
By becoming less shy, my observable behavior would be more extroverted, but I wouldn't be any more extroverted than I was originally, since some of what's making me a quiet person now is attributable to shyness rather than introversion. Unfortunately, these concepts are very difficult to tease apart, and it's tough for me to tell whether my behavior in a given situation is due to shyness, introversion, or both.
As for extroverts becoming more introverted as they age, I think much of this has to do with the physical aging process that necessitates slowing down, as Marti stated in her book.