From the bones alone the FA can determine the age of the individual to within a matter of weeks if it's a baby; within 2-3 years if it's a young child; within about 5 years during adolescence, and then the reliability for age determination decreases markedly for adults.  

In the early stages of adulthood, age can be determined to within 5 years but this increases to within 10 years for mature adults and then to within 20 or more for older adults.

The changes that are seen in the child’s skeleton relate to growth which happens at a regular rate but the changes that occur in the adult are a result of degeneration and we all start to break down at different rates.