I was born in 1985, so obviously I couldn't have been a fan since Boy.
The earliest U2 song I remember is Desire (which I didn't actually identify as a U2 song until much later). The only other U2 song I remember before starting to listen to U2 is Mysterious Ways.
I was first aware of U2 when they came to Chicago in the summer of 1997. Two of my older brothers are huge fans (though I think I've surpassed them). I lived downtown with my folks and my little brother, and my two older brothers (who lived in the suburbs with their mom) came into town to see U2 at Soldier Field, a little under a mile away. Nobody thought to themselves, "Gosh, I bet Gavin would love this," so I wound up only being aware that my brothers were off to see U2. I guess I knew they were a rock band, and I was able to hear a little noise from the concert that night, but I wasn't interested.
For Christmas in 1999, I was looking for a gift for one of the afforementioned older brothers. Knowing of their U2 fandom, when I came across the Best Of 1980-1990, I bought it, thinking it was perfect. Of course, he already had it, but I didn't know that. He didn't show up for Christmas, either, so I never gave it to him. It stayed on a shelf in my room for months.
One day in the fall of 2000, I was looking for something to listen to. At the time, the only music radio I EVER listened to was the oldies station at night. Remembering the U2 CD I had bought for my brother, I finally figured that he hadn't shown up to claim it, so it was rightfully mine. I put in my CD player.
And I was hooked.
I didn't listen carefully. I never do on the first listen of anything. But I knew I loved what I was listening to. I started carrying my CD player around with me, and listened to the Best Of and the B-Sides almost constantly on the train to and from school. I had no idea that U2 was completing a new album, and about to go on tour. I bought three albums. First, the Joshua Tree. Then, on the recommendation of my other older brother, the Unforgettable Fire and Achtung Baby. That's where my collection stood, actually, until relatively recently.
Knowing I was a burgeoning U2 fan, the same brother who had been guiding me through the albums bought me Rattle and Hum on DVD. The second I saw "Where The Streets Have No Name", I was a U2 fan for life. This was in the Spring of 2001, after the Elevation tour swung through Chicago (a fact I was not aware of, anyway, so it's not like I missed it).
Through the summer, though, I loved every second of U2. I heard a few of the new songs on the radio, and loved those, too. I remember downloading Beautiful Day and Elevation (and Walk On, too, I think). Having seen Rattle and Hum, I knew what an amazing live band they were, so you can imagine my elation when I read that they'd be touring North America, including Chicago.
I had my dad buy a ticket for me when they went on sale (I was in school, remember). I instructed him rather clearly to buy me the cheap floor ticket ($45 GA), but he decided that if he were going (he wasn't), he'd want a seat. That's how I wound up sitting on the opposite side of the United Center from U2. Not that it really mattered to me; the show was amazing, I heard some songs I hadn't heard before (Wake Up Dead Man, Kite, Stay, Peace On Earth), and generally had an amazing time. Since then, I've been listening to U2 a lot on and off, intermixed with periods of heavy Weezer, Beatles, and Nirvana listening. My girlfriend gave me the Best of 1990-2000 for Christmas a couple years back.
Vertigo's release jumped me out of my Weezer listening, and since then, I've bought Boy, War, and How To Dismantle An Atomic Bomb. The albums I haven't bought yet I've been heavily exposed to by my older brother and various concert bootlegs.
So yeah, that turned into a long story. But to put it shortly, I'm too young to have been a longtime fan. But I love 'em just as much anyway.