I have noticed this. It seems like you can tell when a plant has reached full maturity. It will start to show its sex in the veg stage and the nodes may start to alternate. When you put that thing into 12/12 it will start to flower almost immediately.
On the other hand when you put a younger plant with an undetermined sex (lets say it was vegged for 2-3 weeks) into 12/12 it will not begin to flower or even show its sex for a week or most likely two. I have also noticed that it will grow much slower in that first week of 12/12 as opposed to the more mature plants. I believe this is because the plants are not yet mature enough to flower but they do so only when THEY are ready.
oh and goooood morning Lacy, ive got myself a nice warm cup of tea and im sitting by my fireplace, its real nice and cozy. Its actually so cozy I may go back to bed since I dotn have work today