they look younger then 6 months old and by that age they would be attempting to mate with a hen I would think. Strange. And they should be crowing by that age too. I'm no expert though. Unfortunately it sounds like you will need to rehome them if they start to crow.
You said they are 6months old, have they tried mating with your hen? -how old is she?... Not nice if you have been lied too about their sex. Silkies are hard to tell sometimes though.
Fyi, Silkies can take up to a year to start laying although a lot do start around 6-7 months.