I've heard of putting clones into 12/12 just after root formation, but a seedling has to mature before it can properly flower.
A clone, theoretically, is at the same level of maturity as the donor mom, so has a bit of a head-start on the process.

If you put seedlings into 12/12 too soon, they just keep vegging very slowly, (get's wrong nutes for further vegging, wrong light spectrum and combative light cycles) till they are ready for flowering. Big waste of time, lower yeild, subject to stress. Unless forced at gunpoint to start your sedlings on 12/12 early, much better to properly veg them till they are biologically ready.