At 4 I'd call it great odds.

At 6 I'd begin to question, if it came from a clutch that might theoretically yield such an animal.

At 7 or 8 I'd think 'hey, this thing very well might a homozygous dominant!'

At 10 I'd feel sure, but uneasy enough to label it as such to the public if no such animal had been produced as of yet. 1 in 1024 is pretty tiny, and far beyond "a reasonable doubt" in my mind.

15, I'd definitely label it as such.