I didn't see any behavior of either bird that was gender indicative?one more question. I was watching my "girls" a minute ago... is it possible to have a cockerel who looks like a pullet even at 10 months and doesn't crow? The splash "pullet" I have acts very boyish and doesn't lay eggs, but hasn't crowed even when egged on by other roosters. Looking at "her" tail today too it seems she has gotten even more stiff tail feathers since the last time I looked. I will post a video of them while they are eating now so you can maybe see what I mean by acting boyish.
Yes, a bird can be older than 8 months and you still not sure of gender. I've had more than one that was older than that (one didn't lay until she was over 13 months, and a boy did not begin developing male features until he was almost 10 months. Just this weekend I spoke with a breeder who's white pullet turned into a cockerel at about 11 months. This is someone who knows chickens; not just silkies; a lot of breeds.