I've been raising silkies for 4 years now and I don't even attempt to sex them for 90 days. Even then I can get it wrong...they're odd little things (but I love them anyway!) Even my standard breeds I let mature about the same amount of time, I'd rather be 99% certain that I'm selling a pullet than have the new owner come back in a month wanting to trade for an actual egg layer instead of the "girl" that started to crow. I sell straight run too, and I make sure the buyer understands that they'll get boys AND girls. You're better off waiting or getting older pullets from someone for a little more money or finding someone who will hold them for you for at least another month. JMO of course.....