When does flowering start

So i was wondering what people think? does flowering start right when you put the lights to 12-12 or when the plants start to actually flower, or when it actually starts to show the sex of the plant?

sometimes plants show signs of sex before you switch to 12/12 so imo when you switch to 12/12 and you see signs of more stigmas (hairs) forming, that will indicate that the plant has induced flowering