When it comes to plant biology the hormones that react to a period of darkness that induces flowering are present in plants from the bean stage on so they could be started off with a 12/12 cycle, but it would not in most cases be at all beneficial.
　
If one were to do it, it would be wise to have either two different lights burning at once, one MH and one HPS (if using HID lighting that is) and have them both set to a 12/12 cycle or use a dual arc bulb (if you use HID lighting in a wattage dual arc bulbs can be purchased in) or with CFL's having the proper combination of Kelvin temperature/color rated bulbs to give an equal mixture of light spectrums. In each case the combination of light spectrums would cut down on the stretching a young plant will have if only under an HPS or other form of lighting with a light spectrum only or mainly for the flowering period. It would not totally eliminate it but it would cut it way down.
　
Bud production would be minimal in most cases so that is again something not beneficial but to be technical you can never flower to early as in it being harmful to the plant itself. It will only be harmful to the plants productions which actually means it is harmful to you and not the plant. It is extremely unwise in most cases to begin plants off with a 12/12 light cycle due to the number of negatives that go with doing so.
　
In the case of growing a pure or nearly pure sativa that will grow as tall as a sequoia tree indoor growers will often start them off with a 12/12 light cycle. Giving one even a short vegetative period of growth will almost assure that without massive lighting, major headroom or lots of plant training and or topping the plants will quickly outgrow the growers light capabilities and also all usable growing height.
　
As usual I took the long way around the barn to simply say if one properly and only when needed starts out their plants on a 12/12 light cycle the plants themselves will not suffer any negative effects. If done improperly, and when not needed, all that someone will receive is negative effects. It will not change a plants sex or cause hermies all on its own but what is seen in the pictures when it comes to growth and nodal spacing and evident stem weakness etc. pretty much tells most of the story.
　
Don’t ever do it unless you absolutely have to do it and only then if you know how to do it in the best possible way.
