Gender of my guppies?

I have a 10 gal tank with about 15 baby guppies, and some are indeed older than others. I was wondering, since sone have spread out tails, and the others tails are pinched together, is this Judy because some are older? Or is it how tell their gender? They don't have their colors yet and I can't see their gravid spots ( females ) if so the ones with the pinched tails are what gender? The oldest babies are just over a month, youngest are a few weeks?