What is the earliest sci-fi story you remember reading, and what age were you?
An interesting question, mostly because I can't actually remember the answer. It is kind of like 'when did you first realise you were gay?', 'when did you first realise you were skiffy?'
Clearly, I must have somehow gravitated to the SF book section at the library. I think the SF books may well have had a convenient marker like having a blue sticker on the spines or something like that. Also, my brother had SF books so I got to read e.g. Vonnegut and A Hamstercle for Weaselwitz at an early age. The comment I always make is that I got into SF via William Burroughs rather than Edgar Rice Burroughs but that is actually a bit of an exaggeration since I didn't really like the William Burroughs at the time, although it was them that were in the house not the ER :-)
Early things I remember reading... The John Christopher, Heinlein (Between Planets, Space Family Stone, Citizen of the Galaxy), James Blish (particularly the Heinlein Pastiche). Leonard Wibberley (alias Patrick O'Connor).
My recollections of Wibberley / O'Connor suggests that I was really only reading SF by that point, because I remember reading one of his non-SF books only because it was by someone who wrote SF - as I may have said, I have since bought some of his books by the miracle of Abebooks. The Wikipedia article about him is very poor, this may be the thing that finally encourages me to edit something (apart from removing a bit of anti-semitism).