12 hour light effect on young plants

I have 2 sets of seeds growing in hydro. Unfortunately the 2nd set got started 3 weeks later than the first set.

The first plants are ready to go into 12 hour light cycle.

I really don't have a good place to separate the two groups of plants.

Will it harm my younger plants to put them on only 12 hours of light? Actually I guess I should ask, what will happen to them? Will they just take longer to get to the flowering stage or will they go in prematurely and not really produce?

Thanks for the help!

your going to have to separate them. I have never been put in that situation so i cant tell you first hand but i have read a few times that it will begin to flower your plant.