Well, I'm reminded of a scene from Morning Musume DVD Magazine Vol 12 (i.e., backstage of the 2006 Autumn tour) when Koharu asked the same question and Sayu told her they counted from first singles.
Yeah, it's unofficial, but it makes more sense to me. Different people come to the business in different ways, and they spend much different amounts of time preparing for their major debuts. That first single is the only fair and truly objective measure.