Yea you can throw them into 12/12 to see if they have female flowers...But i think its better just to let them grow out, and when they start having alternating nodes (staggered branches) they'll usually start growing pre-flowers.
I had a seedling that was 4 weeks old, just under 18/6, and it showed pre-flowers.
Judging by the pictures, i'd give that another 2-3 weeks until you start throwing nutrients at it.