Even different books say diffrent things, but I can tell you from BOOKS and experience. An ADULT PLant, a MATURE Plant can show sex and Pre- flowers after 4 and half weeks from germination.
When a plant is 5 weeks old, even if still under 24 hour light, it will show pre-flowers. NOT flowers, but pre-flowers. 12/12 forces flowering, even on immature plants. But flowering will begin when a plant becoems an adult.
Pot plants make perfectly opposing nodes, one leaf directly across from another until it is an adult. Then it will start making them staggered, or one leaf a little higher or lower than the opposing leaf. THEN it can show sex.