you can really do it whenever you want...i know people who flower as soon as the plant pops outa the soil. id say the best is to wait till it gets like 5 or 6 nodes and has a good stem and root system. but keep in mind that longer you let it veg the more yield you get. unless your on a time or a space limit.
PEACE
how old and tall does a plant haft to be before you can change lighting and try to show the sex 