In the LA Times there's a story about using X-rays of bones to estimate people's age. The reporter's talking about the potential for this technique to obtain fairly accurate ages for those tiny, brilliant, and possibly under-age Chinese gymnasts from the 2008 Olympics. But the underlying anatomical and developmental data have been applied to some equally problematic, but much older, remains - those of the Turkana boy.
It's possible to get an accurate idea of someone's age from examining their X-rays because the skeleton ossifies, and individual bones fuse, to a predictable schedule. You're probably all familiar with the fact that a human infant's skull has a couple of 'soft spots', or fontanelles, where the bones have yet to form & fuse. Similarly, the bones of a newborn are only partly ossified (in other words, bits of them are still cartilagenous), and they aren't all one piece: the ends of a bone like the femur are separated from the shaft at the growth plates. The ends & shafts of leg bones begin to fuse at around 15 years old for European girls, and the same thing happens in the elbow at 13-14. The ages at which this happens is likely to be a bit different in the young Chinese gymnasts, but there are growth tables available for Asians, so the comparison would be fairly straightforward. You can do the same thing with teeth, of course, because there's a predictable schedule for the eruption and loss of 'baby' (deciduous) teeth & their replacement with adult teeth. Wisdom teeth, for example, are fully formed by about the time you're 19. Combining the data from both bones & teeth will give you a more accurate idea of someone's age than using the data from one source alone.
And how is this related to the Turkana boy? The 'boy' is a Homo erectus individual, a fossil discovered in 1984 by Kamoya Kimeu (expedition foreman for Richard Leakey and Alan Walker). One of the wonderful things about this fossil is its completeness - almost everything is there, apart from the bones of the hands and feet. (Given that for an individual to become a fossil is a rare event, and being in the right place & time to find a fossil as it erodes out of the substrate is even rarer, the Turkana boy really is exceptional.)
The skull was the first part of the skeleton to be found & pieced together. Among other things, the research team noticed that
there were no sockets for the last molars, the wisdom teeth - meaning that we were in possession of an adolescent Homo erectus skull, a rare find... This was why the vault bones [of the cranium] were a bit thin and why they had come apart so cleanly at the sutures, the points where one bone grows to meet its partner. Conveniently for palaeontologists, skull sutures stay open - cartilagenous rather than bony - through childhood. (Walker & Shipman, 1996).
The question then was, just how old was the Turkana boy when he died? The postcranial skeleton would help to answer this. When a tibia (the main bone of the lower leg) was found, it became obvious that they were probably going to find a lot of the skeleton. Then the team knew that they would be able to use the degree of fusion between the shafts of long bones and the growth centres (epiphyses) at the bones' ends to get a closer estimate of his age. (Detailed measurements of his skull and pelvis had already confirmed his gender.) It turned out that in none of the long bones were all the shafts and epiphyses fused, which meant that if he grew at the same rate as a modern human he had to be less than 16 or 17. The degree of fusion in his elbow & pelvis suggested that he was probably under 15 years old, & maybe less than 13.
But there's still that 'if', because we don't actually know how fast erectus individuals grew. The boy's teeth helped to determine this, in a series of comparisons between modern humans, erectus, and chimpanzees that found that
the boy differed from humans in the early eruption of the molar teeth; he differed, rather widely, from the chimp or great ape model in the very late eruption of the canine teeth. In other words, he had not yet evolved a human pattern of dental development, but he seemed to be moving toward one and away from the apelike condition that is presumed to be ancestral (Walker & Shipman, 1996).
This analysis suggested an age between 11 and 13. And a fairly tall 11-13 year-old, because as the skeleton was pieced together it became clear he'd have been about 158cm (5ft 3 inches) tall. This gave a projected adult height of around 183cm, which fitted in with the other known erectus remains.
Forensic palaeontology, anyone?
A. Walker & P. Shipman (1996) The wisdom of bones: in search of human origins. Orion Books, UK.