Everytime my son has a bone age, the bones in his hands show several different ages.   At first the range was no more than 6 month apart.   Then some years later 4 years apart.    I don’t think it is uncommon to have six month differences, and generally the range between age should not be any MORE than  than 6 months.  However, when it exceeds this, you really need to take the oldest age given on a scan and go with it to be honest.  It will be accurate give or take a difference of 6 months only, and if your child has bone ages showing anywhere of 11 years at aged 7 years, I would really take that scan seriously and look at what is happening.  Some bones in the hand develop at different rates to others or should I say before others. 

Its also a good idea to stick with the same radiology person every time they do a scan as different radilogisist read the scans differently and it leads to too much confusion which if this delays adequate treatment is with a child for life.