I was about 7, and bought 50 issues until giving up at about 11.
At about. 12, my brother bought me a one off Nightfall story. It was when Batman fights Firefly, when Bane is "wearing him out." I also happened to lay my hands on the Death of Superman, by pure accident really.
Then when I was 14 I started going back and collecting all the GEs and buying every comic I could. If you count the TAS comics, the GEs, the one-stands, and the arcs, I have about 600 Batman comics, and it's still growing.
I still need to get The Cult, Knightfall, and No Mans Land. Oh, and The Man Who Laughs, but that ani't out in England yet. (When I asked my comic guy about it, he was like "Wuhh? You mean Year One?")