I'm catching up here. Dumbledore started to suspect horcruxes when Ginny was possessed by the diary. So it was fairly recent.
We know the original Order was trying to fight Voldemort. We know Voldemort was particularly afraid of Dumbledore. Maybe V was just successful in avoiding him?
Now, here is what else is puzzling me. Harry is, albeit unwittingly, the seventh horcrux. In Hallows, there is some discussion (between Harry and Hermione, I think) about the risk of being too close to a horcrux for any length of time. Even wearing the locket turns out to be risky. And Hermione points out that the reason Ginny got into real trouble was that she opened herself emotionally to the horcrux that was Riddle's diary, a risk also spelled out in Chamber of Secrets. It is made very clear that there is real danger in becoming emotionally close to a horcrux. So, as Harry is the horcrux, how come for seven books, everyone from Dumbledore to Ginny gets so emotionally close to him with no ill effects?