I was thirteen (and two-thirds) when BtVS premiered.
I watched regularly from the very beginning, though I didn't discover fandom until the summer after S4 (I distinctly remember YahooGroups discussion of "Restless").
I learned how to be fannish through this fandom. I have yet to love any show hardcore like I did the Jossverses, and every media text I encounter I expect the same level of meta/fiction/icons/etc. as this fandom has. [Which is not to deny all that I learned about teh crazeh in this fandom either.]
Shared affinity remains a prime way for me to connect with people. [I can name at least two important friendships I have basically because of shared Buffy fannishness, and people automatically win points with me for being fans of any of the Jossverses.]
I still love reading and writing fic in this fandom and still reference life to Buffy and insist that friends should watch the DVDs. I will throw down any time anyone disses my shows of choice, or fannishness more generally.
I don't think of the Jossverses as active movers in my life at the moment but rather as a constant fact of my history/existence. It's sort of weird to so the "how has it changed/affected my life" game when I think of it as such a huge taken-for-granted fact.