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Osteology 101- Part 2/5: Age Estimation

How old are your bones? We give ourselves a birthday every year, so what about our skeletons? One of the things Osteologists do when building a ‘biological profile’ on a skeletal individual is create an age estimate. Having a general idea of how old the individual was that you are analysing can provide you with additional information that can enlighten several of your other skeletal observations. Before we get into the nitty gritty of it all, remember that there is a difference between your biological age and your osteological age. Your bones are the foundation to your entire walking and talking being. They bear a lot of the weight of the (literal) obstacles life can throw at you. We use the term age estimation because we can never fully determine the precise age of an individual. The biological body, with its functions and evolutionary characteristics is one thing. Whereas the concept of time is another social discussion. But that kind of pondering is for another day.

When estimating an age, the other key descriptive terminology to recognise is that we are estimating the individual’s age at the time of death (age-at-death), rather than the time that has elapsed since death. We do this by observing morphological features in different parts of the skeleton and recognising the variabilities between features of skeletons of different ages. Nowadays, we have had scientists of the past do mass studies to determine the trends behind these variabilities, so that today we will typically reference these and categorise these features into age groups. We then determine a general average that places the individual being analysed within one of the categories: fetus (before birth), infant (0-3 years), child (3-12 years), adolescent (12-18 years), young adult (18-35 years; also, can split to early young and early old), middle adult (35-50 years), and older adult (50+ years). The age categories can differ slightly when comparing methods between institutions, however this generally covers the defining terms for different categories. In the grand scheme of things: <18 years of age will be non-adult, and >18 years of age will be adult.

So, what are we looking for? The major markers for ageing are traditionally found in the skull, the long bones, and in the pelvic bones. Though you would be reading separate (and long) papers for each feature we look for, we will skim over those three categories. The skull hosts two types of age estimation features; in cranial suture fusion and in the dentition. Our cranial bones are separated at birth (hence squishy head on newborn babies), and fuse over time. Once these bones meet and begin to fuse together, the stage that they are at can give an osteologist a general idea of whether the person is younger or much older. There are ~17 suture sites to observe from one fully preserved head.

The teeth, on the other hand, can narrow down an age estimate quite well. You can look at tooth eruption times (e.g. losing the milk teeth as a child, and having other teeth grow in its place) for non-adults. Or once all the permanent teeth are in place, you can start to see them wear down. From this there are hundreds of different radiographic, observational, microscopic, macroscopic, etc. methods to determine an age estimate.

Dental Wear: Richter and Eliasson 2017, Erosive and Mechanical Tooth Wear in Viking Age Icelanders. Dentistry Journal 5(3).

The long bones are going to be, well, your longer bones. This considers your arms and legs. Similar to what we had already discussed with cranial suture closure, the epiphyses, or ends, of the long bone need to fuse to the body, or metaphysis and thus diaphysis, of the bone. This occurs at different stages to different long bones which we can recognise, analyse, and categorise. In scientific terms, this method observes epiphyseal closure. Mind you, there are other fusions one can also observe such as that of the sacrum and the fusions of sternal body segments.

Lastly, we have the pelvic girdle. This analysis provides us with slightly more precise information. When ageing via this part of the body, the two major components we look at are the pubic symphyseal surface and the auricular surface of the ilium. The pubic symphyseal surface changes in texture and form well into adulthood. Therefore, even when you are of an ‘older adult’, we can still observe the metamorphosis of this surface. When a person is younger, this surface will be rough and rugged. As a person ages, this surface loses the intensity of the ridges and grooves, is bound by a rim, and just generally deteriorates. The auricular surface of the ilium follows the same protocol. A younger individual will have a more regular granulated, porous surface, with patterned features such as billowing and striations. These characteristics become coarser, and the transverse organization of these striating features becomes more inconsistent. This style of observing such texture and feature changes in surfaces can also be applied to the sternal rib end, particularly the fourth rib.

Although we could talk for hours about this single topic, the major takeaway is that we always want to look at as much as possible to determine a general age estimate. Changes in a skeletal structure are relative to so many things, and it’s virtually impossible to determine why each individual may differ from the next one regarding external factors during their time alive. Additionally, having a perfectly preserved skeleton in archaeological sites, is a treat (creepy, yes) but rare. Therefore, we have to make do with the fragmentary remains we are confronted with. In line with this matter, these methods are always being tweaked and improved. We strive for accuracy and precision, and there is always room for improvement!

Thanks again to Jude to writing this piece and being the awesome lady that she is! Also huge thank you to Leiden University for letting us film at the Laboratory for Human Osteoarchaeology. For more information, click here to visit their website.

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