Onyx, at 6 months old... I can confidently say is a pullet... And NOT because she hasn't crowed as that is an individual personality thing and my friend had a very clear LF cockerel... who didn't make his first crow until almost 2 years old! Then suddenly this season he found his voice.
That gal there though... she isn't sporting a mullet, sometimes referred to as streamers when first coming in. But she also does NOT have the long saddle feathers that males will usually be sporting quite well by this age... even Silkies. Stance is also pullet style. So while none of these clues may mean anything by themselves... added together, I think it's a clear picture!
How late they lay will depend on several things including line, feed, day light hours, time of year they hatch, etc... I have had my line lay as early as about 20 weeks when hatched super early in spring and as late as 8 months. I have heard tell of others not laying until maybe a year old or more! And I have had other breeds not lay until 9+ months, that went through mini molts or what have you despite being provided for properly... they do what their hormones tell them not what our books say they should.
I also have to wonder if breeds with large crests are getting light blocked out extra and delaying onset of lay.

But 24 weeks (or 6 months) is still a tad early this time of year in my experience... for Silkies.