Good thread.
1) The Beatles (age 5) -- Cemented my love of music. What more can I add? 'Nuff said.
2) The Smiths (age 11) -- First band that made me realize I could find an identity through music...just be yourself. Be geeky, be strange, be uncool, be queer, be miserable, be happy, but be true to yourself.
3) The Pixies (age 14) -- The band that made my world of black and white go to technicolor. They completely changed how I listened to music, they opened up a door that I walked through, closed, locked and threw away the key...first concert I ever saw, and first band that I really felt was "my own". I was ridiculed for liking "the Pussies" by the numbskulls that listened to Metallica and Ice T. I guarantee you later on those people went to college and bought "Last Splash" by the Breeders to make a girl like them or listened to Pavement (okay, Pearl Jam). But the Pixies made some of the most difficult years of life bearable.
4) Elliott Smith (age 23) -- Technically not a band, but screw it, it's my list. Plus, he toured with a band and got his start in Heatmiser. One of those voices and songwriters that made me realize I was growing up and needed another voice, another direction. Still one of the most special people to ever grasp a guitar and stitch together words, music, and emotion into the most remarkable form of beauty and art.
5) Luna (age 34) -- I got to see Luna in the 90's but it wasn't until after they broke up that I really explored their discography and realized their extraordinary genius and that includes their last LP "Rendezvous". Sometimes it takes life experience to really appreciate the intricacy and detail of certain music. For me, Luna represents a step into adulthood where I finally became more secure in who and what I am. I discovered the bulk of their music at a time when I realized that I only have myself...I'm the one who chooses what I want and need from life. Seeing their tour last autumn was magical to me. I still keep discovering music that I obsess over and that makes me hear things differently. But I'm still a picky mofo.