I've heard many accounts of folks starting a seed at 12/12, meaning it IS possible, although the plant won't initiate flowering until it's mature enough to do so. But unless you're planning to run a SOG, with numerous plants, you'll be kicking yourself in the ass over yield. I gotta agree with Rusty, you'll get a better yield if you at least have SOME vegging... preferably enough to let the plants get at least four or five nodes on them, then switch to flower lighting. If height is an issue, train or top 'em to keep the canopy even and low.