The first set I remember having was the Test Car (8865) so I was around 6-7 years old then. Loved the pop up headlights and the moving seats. The last set I remember having was the Technic Space Shuttle (8480), so it was probably around 1997 that I stopped playing properly with it (aged 15). In between that I always had lots of Pirate, Castle, Space and Underwater lego, but Technic was definitely my favourite.
My biggest regret is getting rid of it all - I had massive bucket loads but can't even remember where most of it went, but I do (shamefully) remember throwing the Shuttle away in the wheelie bin when I was about 19 and clearing out of my room. Still can't believe I did it, especially now I'm building up a collection again.
My recent purchases started October this year after a trip into the Downtown Disney store while on holiday made me want the Supercar (8070), so I guess my dark age was 1997-2012.