What is bone age?
Bone age (skeletal age) is the measure of how mature a child’s bones are. Usually an X-ray of the left hand and wrist is taken, and the appearance of the ossification centres and growth plates is compared with standard references. The result is expressed as an “age” and can be ahead of or behind the child’s calendar age.
Bone age is accepted as the gold standard of skeletal maturation because it shows bone development directly. While calendar age tells you how long the child has been alive, bone age tells you where their body is in its growth journey.
How is it assessed?
There are two classic methods for determining bone age. In the Greulich-Pyle method, the X-ray is compared with the standard images of an atlas and the most similar age is chosen. In the Tanner-Whitehouse method, individual bones are given a maturity score and the total score is converted into an age. In modern practice, AI-assisted systems can also help with this assessment; but the final interpretation is made in the clinical context.
The difference between bone age and calendar age is informative: a delayed bone age means “there is still growth in the hand”, while an advanced bone age suggests growth may finish earlier than expected.
Bone age and height prediction
One of the most powerful uses of bone age is adult-height prediction. The Bayley-Pinneau method uses the current height and bone age to estimate what percentage of their adult height a child has reached, and therefore how much more they will grow. In a late-developing child (delayed bone age) this method is usually reassuring: growth can continue even after peers have stopped.
Bone age is also used in distinguishing precocious and delayed puberty, in evaluating certain growth disorders, and in maturation-based planning in sport. It does not diagnose on its own; it is interpreted together with growth velocity, height and the clinical picture.
Is an X-ray necessary?
Bone age requires a low-dose hand-wrist X-ray. This is requested at a doctor’s decision when there is a clinical indication — for example marked short/tall stature, a question about pubertal timing, or a need for height prediction. It is not taken for every child as a routine screening.
There are also alternatives that do not require an X-ray: the Khamis-Roche method predicts adult height from age, height, weight and parental height; indicators such as %PAH estimate maturation without radiation. These tools provide a practical starting point in situations where an X-ray is not necessary.
Reading the bone age – calendar age difference
The difference between bone age and calendar age is not a single grade on its own but a piece of context. Differences of a year or two are common in children and usually normal: a delayed bone age usually points to a late-developing child with more growth ahead; an advanced bone age points to an early-developing child closer to finishing their growth. This difference also explains why height prediction cannot be made by looking at current height alone.
What matters is interpreting this difference together with growth velocity, height percentile and pubertal signs. For example, in a short child with a delayed bone age the picture is usually reassuring; short stature together with an advanced bone age is assessed more carefully. Bone age is part of the bigger picture; it is not used as a diagnostic tool on its own and its interpretation belongs to the doctor.