I think my very first forays into programming were around 7th or 8th grade, so about 10 or 11 years old. But it didn't really "hook" me then, maybe in part because I didn't have a proper computer... just a "Bally Arcade" game console with a BASIC cartridge you could plug in. But without a keyboard, entering code was really tedious and painful.
When I really think of myself as "getting into programming big time" was my first year of college. I would have been 18 then. I took a couple of programming classes as electives, and "the bug bit" as they say, and I bought the parts to build a PC (386DX40 FTW!) and downloaded a shareware C compiler from a BBS, bought Herbert Schildt's book Teach Yourself C and just dove in... never looked back. :-)