The premise of DC is that Kudou Shinichi, age 17 (or sixteen, the early manga chapters kinda wobble on that) is fed a poison that de-ages him by six years, making him Edogawa Conan, age seven (or six.) If Haibara's been working seven years for the cure, that makes Conan fourteen and Kaito twenty-four. The series itself runs on comic-book time, and so no time has passed while fifteen years have come and gone. Shinichi is still chibi and will be 'til the end of the series. :)