I can confirm that switching a plant to 12/12 at any age will trigger flowering. I had some seedlings that I kept in the flowering room as I did not have a seperate veg room. I grew the seedlings like this until they were about 4 nodes tall. I was scared they would flower so I built a veg room. They have been on 24/0 since then. I noticed at about the 5th node that males were producing male pre-flowers aka balls and that the females had 2 hairs on the highest node. So far flowering has appeared to stop but I am not 100% sure that it will.
Hey, i am in the same boat right now. I am going to have the seedlings in the flower room for about 10-12 days, do you think I can switch them to the veg room after that and they will not flower? How did your's go? Mainly because I want to veg these out and make them into the mothers.