Having described the conceptual framework we're using for the course, this lecture will now focus on the different definitions that are used to define adolescence. Let's start by thinking about childhood and how we actually define it. Now, you could be forgiven for thinking that childhood stops at five in many parts of the world, given how much of current global child health policy, has been directed at the under five year old space. For example, in millennium development goal number four is to reduce child mortality, but this is defined as reducing mortality in children under the age of five. Now, given the extraordinarily high rates of mortality in children under the age of five, and particularly in infants, this is entirely appropriate. But I suggest that we should be referring to this goal as a reduction for of under-five mortality, and not by the shorthand of reducing child mortality. Now in my own country, Australia, much data about children is reported for 0 to 14 year olds, with data about older children commonly being lumped together with older adolescents, and indeed with much older adults such as data that, for example, refers to 15 to 45 year olds in one data grouping, or data referring to 15 to 65 year olds in other reports. The UN convention, on the rights of the child, includes children up to 18 years of age, and in many parts of the world, 18 years is thought to mark the legal end of childhood. Now, certainly it is the age that, in many parts of the world, they first get the right to vote. But different ages for different laws suggest that the age of legal maturity is much more gray than the black and white that a line like this, between children and adults would suggest. So what then about the different terms and definitions that have defined adolescence? Now, the term adolescence was defined by the WHO, the World Health Organization, as that period between 10 and 19 years of age. In contrast, youth has been defined by the United Nations as people between 15 and 24 years of age. And the combination of these terms, that is adolescents and youth, spans the 10 to 24 year old age group. And, we refer to this period as adolescents and young adults, and commonly you're going to using the shorthand of adolescents to refer to this expanded age group of 10 up to 24. We're using this age group in some ways because it builds on the arguments that we made for the second series of adolescent health that was published in the Lancet in 2012, where we include data about this. And we'll be including references to these publications across the course. However, I think that importantly you must recognize there's nothing intrinsically set or special about any of these definitions, which were no doubt chosen because they were simply consistent with the five year age bands that are commonly used by demographers, 0 to 1, 1 to 4, 5 to 9, 10 to 14, 15 to 24. However, given the changing shape of adolescence starting earlier and ending later, this extended period of years from 10 up to 24 now seems perfectly appropriate. Now, perhaps in the future, we might have much more precise markers of adulthood. Who knows? Markers of adolescent brain development, in say another 30 years time, might denote adult brain functioning, but certainly we don't at the moment. Now in some ways, there's nothing new about this expanded age rage of adolescence. For example, Shakespeare had his Shepard in the Winter's Tale say, I would there were no age between ten and three and twenty, or that youth would sleep out the rest, for there is nothing in the between but getting wenches with child, that's servants getting pregnant, wronging the ancientry, that's the old folk I gather, stealing and fighting. Now, note that Shakespeare's focus was 10 and 23, very similar to what we're describing in this course. And clearly, he viewed adolescence as a time of significant risk. Whereas Baron Colmar von der Goltz, a Prussian military general who was born in 1843, was very clear about a very different set of opportunities provided by adolescence. And in his fascinating treatise on military art and science, The Nation in Arms, he described, I think, incredibly eloquently, that the 18th to the 24th year is best suited to military service. The body is then quite vigorous enough to endure hardships. If you like, it's physically mature. And the soldier is as yet free and unfettered, and I take this to mean that there's not yet a marriage or mortgage. He describes them, the grain of heedlessness, a quality peculiar to the freshness of youth, is an excellent incentive to martial achievement. Now, his notion of that grain of heedlessness is absolutely spot on with advances from contemporary neuroscience, which suggests that maturation of the adolescent brain continues at least into the mid 20s. Although I must say that brain maturation seems far less eloquent a term than von der Goltz's grain of heedlessness, but don't let my neuroscientist colleagues hear me say that. Hang around, though, because later on in this course we'll be reviewing adolescent brain development in much more detail. But let's think about the consequences of these definitions of adolescence and their health. And let's think about what this means in terms of the visibility of the young. Now what image comes to mind when you think of a child? What image comes to mind when you think of an adolescent? When you think of a young person? What image comes to mind when you think of an adult? Can I ask you to look at these photos? And think about definitions of children, of adolescents, of adults. Because even if formal definitions of childhood extend up to 18 years, in this picture on the left, I would have thought if I was asking you what image comes to mind when you look at this picture of a child, most of you might focus on the child that's sitting on the lap of the woman on the right, or perhaps the person in the middle. But what would you say if I were to tell you that all of these young women on the left photo were children? And if I were to ask you in the middle photo, what image comes to mind? Or which of these two figures of the middle photo is the child, I'm sure you would immediately look to the infant on this mother's lap. Now, might you think differently about our responsibilities as a society to the mother in the middle, if we knew she was 15, or 21, or 25? What about this young man on the right? I would argue that inconsistent definitions of adolescence and young adulthood, and inconsistency in how data that relates to adolescence and young adults are reported at local, national, and global levels, is one practice that makes adolescents and their health invisible. And adolescents and their health shouldn't be invisible. In summary, there is no fixed age that defines adolescence, whose onset has been historically defined by the biological features of puberty, and it ends with a series of social role transitions that are far less linear and fixed now than previously. Adolescence now describes a much longer phase within the life course than previously, and notwithstanding historical definitions. In this course, we're going to preferentially use the term adolescence and young adults to refer to the 10 to 24 year old age span. But, no doubt we will interchangeably use the language of adolescence, youth, young people, adolescents, and young adults, in referring to this age span.