This is a topic that comes up frequently on the SDS Support loop. My youngest SDS son had a mild bone age delay -- at 8.5 he had a mild delay of 3 years. My middle son has a 5-6 year bone age dealy at the age of 10.5.
Here are a few links that might be helpful:
Bone Age test
Wrist x-ray and Bone Age Info
Bone Age from Answers.com