A delayed bone age means that the bones think they are younger than the child actually is. This can mean catch up growth after peers have stopped growing. A delayed bone age can happen for many reasons, but a common one is due to late puberty and can run in families. A history, physical exam and possibly labs can help asses the reason for delayed bone age.
If the bone age equals the actual age, you can estimate the final height to be about the same percentage as the current height.
We often repeat bone ages to see if they are changing at a different rate than the child grows.
In the boy growth charts pictured below, the top set of lines is the height (stature) for age chart. The bottom set is the weight chart. The ages that are used to plot a height at a given time are listed at the top and bottom. The heights are listed on the right and left of the graph. The middle line is 50%, which reflects a height of 50% (taller than half/shorter than half of boys of the same age). The other lines are also labeled for their respective height percentiles on the far right. Hopefully this looks familiar because you've seen a graph like this at your child's doctor. If you haven't, be sure to ask to see it next time you're there.
There are many "normal" heights, usually determined by genetics. Tall parents tend to have tall kids, short parents have short kids. There is no "correct" height or "best" height, the percentiles simply give us a way to follow the growth over time and estimate final adult height if a child hits puberty at a typical age (early puberty stops the growth early, late puberty allows for later growth).
In Figure 1, I filled in a fictitious child's heights with blue dots. You can see that from 3 to 5 years this boy was at the 50th percentile for height. That means he was taller than half the boys his age and shorter than half. The fact that he's in the middle doesn't make it "normal" it just means that if his parents are average height, he is growing as expected because it's consistent year to year and he is of average height like his parents. At 6 years, he dropped to the 25th percentile, and at 8 years he fell to the 10th percentile. This consistent drop in growth often triggers a physician to look for reasons of the drop. Maybe the parents are both very short. Maybe there is a medical problem. Or maybe there is a family history of people having late growth spurts (something called "constitutional growth delay"). Treatment (if needed) varies depending on the cause.
The red arrow on the right marks the actual height at 8 years (blue) at about 47 inches (120cm). For this fictitious child, the bone age is 7 years, and if you plot 47 inches (the actual height at the time) at 7 years (the bone age), you will see this white dot is at the 50th percentile and marked by the red arrow on the left. A delay in bone age often coincides with a late growth spurt. I finished out the growth plots, and this kiddo actually fell more (down to the 5th percentile) before he hit a late puberty and grew into late teens/early 20s to hit a final height at the 50th percentile.
In Figure 2 below a fictitious boy is tall for his age early on. At 6 years old his height (black dot) is at the 97th percentile (he is taller than 97 out of 100 of boys his age) at about 49.5 inches (125.5cm). His bone age at the time (red dot) is 8 years 6 months, which is at the 25th percentile for height. A year later he is off the height chart, taller than over 97% of boys his age, but the bone age is 10 years 9 months, again at the 25th percentile. This chart shows an early growth spurt (as he looks taller than his peers) but an early puberty and a slowed growth faster than other boys. His final height is only at the 25th percentile, much shorter than his early heights would have predicted.
Many parents are super excited when their children are tall and can't comprehend when I talk about the possibility that it might not last. (I typically discuss this if both parents are short but the child is tall, if I see signs of early puberty, or if the child is obese - especially if parents are not as tall as the child's height predicts.) The bone age does not give a reason for the altered growth rate, but can help identify a need for further evaluation and treatment if indicated.
Take home point: At every well visit your child should have a height and weight measured. If the yearly growth accelerates too fast or slows, talk to your doctor about possible reasons. If a bone age is done, you can use a growth chart to put the bone age in at your child's height (instead of actual age) and see how tall the final height estimate would be. It isn't a guarantee, but can be helpful.