I think size more than age determines when to start flowering. the plant needs to be sturdy enough to hold the weight of the buds. It also depends on how much room you have for flowering. Expect the plant to at least double - possibly triple in size during flowering so you may be forced to flower earlier than desirable. You don't want your buds growing right into your lights!
I flower mine at about 12-15" tall. In fact, here is a pic a few days before i went into flowering.... note the 12" ruler.