How early can I sex my plants?

I have a flo light in one grow area, and a halogen in the other. My seedlings that are under the flo have finally entered the vegetative stage. A friend told me that I can flower them this early. Can I? And if so, can I leave them where they are and just go 12/12, or should I move them under the halogen in the other grow area that is already set 12/12 (cuz the plants there are flowering)?

you can flower at about 3-4 weeks from sprout if they have 3+ nodes and some of them are alternating. this means the plants are sexually maure

thanks, that was helpful, but what do you mean by alternating? I understand the part about the nodes, and yes, there are enough. (sorry I'm new at all this).

ill find a picture that describes what i mean better than i can with words

ok this picture is oldschool but youll understand what i mean, the second one is better


notice the leafs are not always on the same side as of the stem

That was very helpful. I get it. Thanks so much.