These next few lectures are concerned with questions about how adolescence has changed over time and how this may have changed patterns of health through the adolescent years. This first lecture tells a story of how adolescence has evolved with a particular focus on the past 200 years. Take a look at these three images and reflect on how growing up might have changed. The first image comes from the 19th century. Adolescent boys working in a factory setting. The second comes from the 1950's, at a time when mass media went through a tremendous phase of growth and youth cultures were becoming much more than local. The last is a much more modern one, and speaks to the global communication and forces shaping adolescent development today. Not just in high income countries but everywhere. These images may have you thinking about the changes in the way adolescents live their lives today. Perhaps you're also thinking about the changes in those forces shaping health and health related lifestyles of young people. But has adolescence, as a phase of life, changed even more fundamentally? I want to now look at how adolescence has emerged as a distinct phase of life over the last 200 years. I want you to think of adolescence as that phase of life that begins with a biological event that is puberty, and traditionally ends with the transition to marriage and parenthood. That biological event of puberty is often marked in girls by menarche or the onset of periods. An event that marks entry into reproductive life. If we go back to the earliest paleontological records at a time before agricultural settlement, when humans were all hunter gatherers, the two events of menarche and parenthood were closely aligned. The gap in girls was around two years, and in boys, the equivalent gap around four years. With the development of agriculture and later moves into towns, the age at menarche increased. Likely to be something to do with increased populations, greater food insecurity, and more childhood infection. But the gap remained around two years between menarche and parenthood. Around 200 years ago at the time of the industrial revolution in the west, this began to shift. Driven partly by extending education, the gap between puberty and parenthood slowly increased. This accelerated in the mid-twentieth century with the development of effective contraception and has extended further in the 21st century. In many high income countries, the gap between puberty and first parenthood approaches two decades. The gap between the onset of sexual activity and marriage or settling in with a life partner can be 15 years. This change in development across the adolescent years has enormous implications for all aspects of health. It has also brought about the emergence of adolescence as a distinct and more substantial part of the life course than ever before. You may be asking, How has this come about? Well firstly, we've seen a fall in the age of the onset of puberty. This chart, drawn mainly from northern European countries, illustrates the changing age of menarche in girls in over the last 150 years. Over that time, it has fallen around four years. The reasons include better nutrition and food security in childhood, and much lower rates of childhood infectious disease. And for boys, similar changes have occurred. What is the difference between a choir boy working under Johan Sebastian Bach in the 18th century, Compared to one in modern times? Prior to the industrial revolution, the average age at which a choir boy's voice broke was 17 and a half years. Today, it's 13 and a half. But that's not all that's changed. The transition to parenthood and marriage is now delayed beyond any earlier point in human history, with effects on fertility And the duration of reproductive life. Young people are spending more time in education. Women are spending longer in the workforce before and after parenting. And young people are not leaving home when they used to. As this slide illustrates, this staying at home, has both social and economic effects on parents, and the young people who are not able to achieve financial independence, with the emotional benefits that this brings. These trends are global. Some aspects, including extending education are almost certainly beneficial for health, but others, much less so. This is a theme we will return to in later lectures. So what is happening to the shape of adolescences in low and middle income countries? Essentially the same thing is happening but it's happening more recently and very much more quickly. It's happening differently in wealthier, urban settings then in poorer remote rural villages. That matters for patterns of adolescent development including the health of young people. In high income countries the process has occurred over 200 years with time for families, communities, education and social structures to adjust. In low and middle income countries, the speed of change challenges all of these social structures and health systems as well as the cultural values that are so central to coherent health and development through the adolescent years. The need to put in place supportive structures for young people who are now very quickly moving toward a different and extended pathway to adulthood is pressing in many low and middle income countries. In part it's about education in schools, but it's also about labour markets and agricultural productivity. It's about families. And family transitions. Social policy in legal frameworks. It's about the non-government institutions, including religious institutions as well as industry and the way industry is marketing to young people. And it's about health services and how they are delivered. These matters are even more significant when one looks at this slide of where adolescents today live. This global map, based on figures from the United Nations population division illustrates those countries with the highest proportion of young people age ten though to 24. In parts of sub-saharan Africa, in Southern Asia ten to 24 year olds make up over a third of the population. In all, just under 90% of the World's young people now live in low and middle income countries. But changing shape of adolescence is now rapidly affecting their health and development, and doing so profoundly. This is a theme that we will return to in future lectures. And we will be looking at how families, social, economic and culture structures and values, essential to the well being of young people, can adapt to this change.