The reading of bone age xrays seems to be very subjective.

My son is five now. When my son was three, his endo read his bone age as two years 8 months. We switched endos mid-year (he was 3 1/2). The new endo read his xray as 3 1/2 to 4 years. A full year difference in the reading from his previous endo.

This year, for his 5 year bone age, they gave several different ages - some bones read 4 years 10 months in some areas of the hand, 6 years in the fingers. They kind of averaged them for the final bone age of 4 years 11 months.

I think it really depends on who is doing the reading.