Yes, the brahma breed and most of the Asiatic class is known to develop at a much slower rate than many other breeds and classes. I believe that it is that way because when they are full grown they are so much bigger and it takes a little longer to build that bone and get up to weight. When I show my brahmas in the under a year classes I try to get them as close to a year as possible without them being a year because unlike your other birds which mature around 6+ months the brahmas are, in my opinion, reaching that mature age around 10+ months. Hope that helps, what you are experiencing is exactly normal for this breed, and good luck to you with them they are a great breed.
-Sam
PS. Brahmas are known for their gentleness.