Made some calculations and you're probably right - about 4 - 5 weeks old. They are very active - trying to fly quite often. Heard that the favourite pet shop breed is the leghorn. I'm assuming this is it. The black one is still a mystery though...
The white ones should be about 1.5 - 2 months old. The black one is much smaller than the white ones and a tiny bit older. Here's another picture of her/him... I agree that the white ones can be cornish x's.