Sexing them has not much to do with age so much as it does size.
I know experts/breeders who can sex them with a loupe at 6 grams, but it gets easier at 8 grams and above.
I personally would not put Cresties together to breed until the female is at least 30+ grams. Before that size, they're still growing and breeding can deplete them of calcium, potentially creating an MBD situation.
Kind of like a teen pregnancy, they can do it, but its hard on a still growing body.
At the very least, you won't get as many healthy viable eggs.
I would keep them separated after about 12 grams.
You run the risk of tail nipping or bullying in any case, so putting two little geckos together can really depend on many factors, like how big the enclosure is, how many hides, and the personalities of the individual geckos.
I don't like to put new ones together with my established ones right away because I like quarantining new animals to make sure they're healthy.
Just my opinion.