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Age can be defined as a simple measure of how long a person or an object has existed. A reliable estimation of the age of a human requires there to be a well-defined and documented event to which the duration of a known passage of time can be ascribed. The thorny question then arises about “when” a human begins to exist. This temporal moment of definition between existence and nonexistence can be difficult to fix in relation to the human and generally has three measurable options: the time since fertilization or intrauterine implantation (which have most value for embryological or fetal aging), or, most commonly, the time since birth for those who survive the uterine environment and a safe passage into the outside world.
Clinical practitioners, but especially pediatricians, wish to know the date of fertilization to permit them to calculate the age of an embryo or a fetus and to assess that development is progressing at the predicted and normal pace. However, unless there has been only one sexual event, then this timing is unlikely to be a certainty, and so clinicians often opt to calculate the age of the fetus from a more reliable event – the last menstrual period (LMP). The mother is more likely to recall or at least approximate this date but of course this can also be unreliable as she may genuinely not remember or may choose to falsify the information for personal reasons. Therefore, even in the earliest stages of human development, age estimation can be imprecise; a fact which is often little understood by the expectant mother who, when given a due date for her baby, is disappointed when that date generally proves to be incorrect. Only some 4–5% of babies arrive on their predicted due date, with approximately 80% arriving in the 2 weeks to either side of that date.
To assign an age to a fetus in utero is generally achieved through ultrasound image measurements of the length of the fetus (crown-rump length), biparietal width of the skull, or length of the long bones, especially those in the lower limb. If the fetus has been miscarried or aborted, then additional measures of weight, head circumference, thorax dimensions, and crown-heel length can be measured. If, however, the remains are skeletonized, then age estimation is normally based on either bone lengths or maturation status of several of the skeletal elements (Cunningham et al. 2016). A fetus can be viable from 23 weeks gestation, but chances of survival are greatly enhanced if it is over 25 weeks. In some contexts, legal abortions of a fetus can be undertaken providing the fetus is less than a prescribed number of weeks of gestation (e.g., in the UK it is less than 24 weeks). Thus, there are two very important forensic questions that can be related to the age of the fetus – (a) was it viable and (b) was the termination legal.
Birth is an eminently definable event in relation to both a time and a specific calendar date. It is perhaps the most important human marker of existence and is celebrated by millions of people as an annual event throughout their entire life. In most western countries, birth is formally recorded through authenticated certification so that “proof” of the date does not rely solely on memory but is supported by verifiable documentation. Indeed, this formal record is so important and desirable that it can carry a high black-market price and may be acquired through fraudulent means. This therefore brings the authenticity of the documented information into some debate, and an understanding that forgeries exist is vital to the realistic acceptance that even documentation may not be relied upon. Many less-developed countries may not record birth dates and therefore have no documented basis upon which to develop a robust certification process. This leads to difficulties when the age of a person is required to be known and there is no formal paperwork to support the believed assumption. Under these circumstances, what is referred to as “chronological age” may be questioned and may require verification for the purposes of border crossings, access to services or legal rights. Government departments, banks, employers, law enforcement agencies, and other legislative authorities require confirmation of the age of the individual with whom they interact, and each can, and frequently does, seek legal recourse to resolve disputes over alleged age assignment (Black et al. 2010). Therefore, the estimation of the age of a person is undeniably of forensic relevance (i.e., pertinent to the courts) irrespective of whether the person is alive or dead. The methodologies and principles followed in both circumstances are very similar.
In the case of a deceased person, an assignment of the age at death is an important component of the biological profile of the individual which can ultimately assist with the identification of the remains in forensic and archaeological investigations. Investigation of a crime is exceptionally difficult in the absence of the identity of the deceased, and indeed investigation of the death is critical for the prosecution of a crime. The biological identity of the deceased is comprised of four principal characteristics – sex, age at death, ancestry, and stature. The personal identity of the deceased can be pursued once these primary characteristics have been attributed. Personal traits of identity may include matching for DNA, fingerprints, dental information, and other, what are deemed to be secondary, indicators of identity including facial appearance. The methods used to estimate age vary little, regardless of whether the deceased is of recent origin or of historical provenance.
Age estimation processes are based on the principle that as time passes (i.e., the further one progresses from the date of birth), then a series of biological maturational events will occur that can be mapped out and cross-referenced to the known occurrence and timing of such events. This requires there to be a strong relationship and correlation between two age-related continua: chronological and biological age. The closer biological age is to chronological age, then the greater will be the accuracy of prediction of one from the other, but the more they diverge, then the more uncertain will be the estimation of chronological age and this will necessitate the provision of a range for the possible age of the individual. The methods utilized in this process tend to show a close and strong relationship between the two ages during childhood, and they start to diverge markedly as adulthood approaches and then move even further apart as middle age approaches and are most divergent into the later decades of human life expectancy. Therefore, the techniques available to forensic anthropologists and osteoarchaeologists permit high levels of accuracy at predicting the age of a child (living or dead), and they are reliable in the later teenage years, acceptable in the early adult years, and very poor in the middle to later years of adult life (Latham and Finnegan 2010).
Biological age has two components, skeletal age and dental age, and each can be established independently or considered in parallel. Skeletal age utilizes information on the timing of formation of bones, their growth, and the pattern of their ultimate fusion to form the final adult product. Almost all growth has ceased in the human skeleton by 30 years of age, and the changes thereafter are degenerative. These are less reliable markers of the passage of time as they are heavily influenced by many factors including genetics, environment, health, nutrition, and life-style. To visualize the skeleton in the living requires that the person usually be exposed to ionizing radiation for imaging purposes (X-ray or computed tomography (CT) scans) although magnetic resonance imaging (MRI) and ultrasound (US) do not carry the same health risks. In the deceased, exposure to radiation for imaging purposes is not as restrictive, and indeed to gain access to the skeleton, soft tissue can be removed through maceration, although of course this does not pose a problem if the remains are partially or completely skeletonized. Age can be estimated to some degree from almost every part of the human skeleton, but obviously in the living, the areas that can be targeted by clinical imaging are restricted and tend to focus on the wrist and, if possible, the medial ends of the clavicle. The radiographic image of the wrist and hand region provides access to at least 29 separate bones, and therefore age estimation can be considered from birth through to middle adolescence with some degree of reliability (Black et al. 2010). Imaging of the medial clavicle permits an indication of age from later teenage years through to early adulthood, but it does carry some health and safety risks in relation to exposure to radiation for imaging purposes. Textbooks exist on how to establish the age of an individual when living (Black et al. 2010), for forensic purposes (Latham and Finnegan 2010) and in the archaeological arena (White and Folkens 2005), but in reality, they all use similar methodologies.
Dental age is predicated on the pattern of known development, eruption, and shedding of teeth that occurs throughout the life of the child (Blenkin 2009). The first tooth to start to develop does so in early embryonic life and the last tooth to reach occlusion does so in early adult life – therefore, accurate age estimation from the teeth is valid from fetal age through to adulthood, but beyond the eruption of the third molars, teeth become less reliable indicators of age (Aka et al. 2016). In the living, teeth are the only hard structures of the body that are visible to the naked eye, and their ability to survive trauma makes them an ideal medium for age evaluation. There is also a largely unsubstantiated, but generally accepted, maxim that dental age is more closely related to chronological age than is skeletal age as the teeth tend to be more protected from environmental insult and so retain a closer parallel progression of the two continua. Therefore, combining estimations of age from the skeleton with those from the teeth will most likely result in an acceptably reliable indicator of age both for the living and the deceased that will be closer to the chronological age than if either were to be used in isolation.
Our modern society demands confirmation of how long we have existed, and the scientist does their best to oblige, but it is never, and can never be, a precise operation. This is the area of research that is most likely to be prevalent in future years as our security-conscious world focuses on who we think we are, who we say we are, and how we can prove it.
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