3rd week of life is true in 12/12 from seed hell even 2nd week sometimes,4-6 weeks for 18/6 -24/0 grows.Usually i spot sex 2nd-3rd week whether it's organs or my techniques.
some times if you veg a plant about 6 weeks up to 8 weeks it will show its sex within that time most of the time and if i put them in to flowring to soon it takes the same time for plants to show pre flowers as it takes for the same plant to flower