I've been a fan since Say My Name. When I watched that video is when I fell in love with Bey. Survivor was the first album I bought, and since then I've purchased every Bey/DC release.
Basically I've been a fan since 2000, so since I was 8 years old.
For the lesser tier of artists, I've supported Carrie since 2005.
Katy, since One of the Boys.
Katharine since 2006.
Alexz since the premier of Instant Star so whatever year that was? 2007? 2006? I don't remember.
Incubus, since Morning View so that's 2002.
Whitney, I've grown up with, so since 88.
Mariah, since 91...
So on, so forth.