Delhi gangrape: How does a bone test determine age?

The Juvenile justice board (JJB) on Monday declared the sixth accused in the Delhi gangrape case as a juvenile.
In the last hearing on 15 January, the headmaster of the school when the accused was a student and the current headmaster, both appeared before the JJB. They submitted a copy of the school register, the only existing document in support of the age of the accused, which shows him to be a minor as of 16 December, 2012.

In cases where age documents are inconclusive, a bone test is done to determine age. Medical experts however say that the bone test can only determine age within a two year range.
Bone age is the degree of maturation of a person’s bones. As one grows through childhood, puberty, and finishes growth as a young adult, the bones change in size and shape. These changes can be seen through x-rays.

Here are five things you should know about how a bone test is done:
*The most commonly used method is based on a single x-ray of the left hand, fingers, and wrist. X-rays of limb bones (pelvic, shoulder, elbow, knee, ankle, thigh, wrist) are used to check how much of the cartilaginous ends of bone have converted into bone, says an Indian Express report
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*Infants have 350 bones and they subsequently fuse into 206 bones by the age of 25. And the different stages of the fusing of bones can give indications of their real age.
* Dental records are commonly used to determine age, since the wearing down of teeth correlates with how long a person has been living and eating. Another indicator is the presence of wisdom teeth.
* Analysis of the pubic bones that lie just under the navel and some vertebral bones can also determine a person’s age. Both the pubic bone and some vertebrae are rougher during the infant ages but start to become smooth as people age.
*These tests cannot determine the age of a person accurately, often giving results in a broad two year range.