for sexing...

I don't think age per say matters after it grows a few sets of leaves, but instead height...like at least 12" to produce any worthwhile amount. Plan ahead for the plants to double in height during flowering to judge when to change your light cycle, while also accounting for the distance you want them from the light. Err on the side of short if it's your first time.

I've read that 18" is a common height to switch, but I started mine flowering when the tallest of 8 plants was 15" because I don't have much height to work with. Within a week, the males budded up. The flowers looked like tiny banana bunches or cauliflower and the females had a few v-shaped hairs. I had to chuck 4 boys.