Why does the professor discuss the "biological clock"?

M: We all change over the course of our adulthood．Some changes we go through are biological and are natural part of aging．Some are cultural experiences, such as the changes related to our family life．A lot c the changes in adulthood involve social relationships and responsibilities．And then, there are some changes that are more internal and personal．Question?
W: Yes．Um, is what you’re talking about-I mean these changes we experience-are they part of our biological clock?
M: I’m glad you brought up the biological clock．The term "biological clock" refers to the biological and chemical changes that occur with aging, as if a clock were ticking away in the background．Some changes in adults are biological．These changes are often easy to observe, like my hair turning gray．Some biological changes aren’t directly visible, such as the ..．uh ..．the reduced efficiency of the neural connections in our brain．But some of the changes we go through have more to do with sociology than biology．There’s also a "social clock" that defines the sequence of normal life experiences, such as ..．uh ..．the timing of education, career, marriage, and so on．Virtually all societies are organized into age strata, periods in life with norms for-that is, typical ..．uh ..．expectations, demands, social roles and responsibilities．People have different expectations of-and different attitudes toward-20-year-Ids, 40-year-olds, and 70-year-olds．We generally expec a 20-year-old to act in a certain way, but we expect different behavior from someone who’s 40 or 70．Our attitudes towards each age group form what we consider to be the age norms for that group.
W: Excuse me, Dr．Butler．Wouldn’t these age norms be different in different cultures? I mean, in some cultures someone who’s 40 is considered old, but in other cultures 40 is still kind of young.
M: Various cultures might define "old" and "young" differently, but every culture has a set of attitudes age norms-for old people and a different set of attitudes for young people．For example, an older adult might be seen as having more-or less-status than a young adult-no matter how the culture defines what age is "old." Early adulthood is the period from age 18 to 40．Early adulthood is when more new social roles are acquired than at any other time of life．The first new role is often independent adulthood, as the young person leaves home．Another new role is worker, as the young adult begin a career．Young adults also acquire the role of spouse, as most first marriages occur during early adulthood．A major new role is that of parent．Over 80 percent of adults in North America will eventually become parents, normally in their twenties or thirties．The second stage of adulthood, middle adulthood, is roughly from age 40 to 65．In middle adulthood, the same social roles are still present-spouse, parent, worker-but they become less demanding and less confining than they were during early adulthood．A few new roles are added, like grandparent, or maybe caregiver to our own aging parents．This is the "sandwich generation," because people in middle adulthood are sandwiched between the needs of their children and the needs of their aging parents．Late adulthood begins at age 65．Late adulthood is normally marked by a decline in the number of social roles．The role of worker is shed at retirement．The older adult is still a parent, but that role now has fewer duties．For many people, the role of spouse is given up to widowhood．Yes?
W: So far, you’ve been talking about people who have children．What about people who don’t have children?
M: Adults who don’t have children don’t experience the rote changes that accompany a child’s development, yet their experiences do not otherwise differ in any substantial way．They follow the basic pattern of adulthood．By this ! mean, they add new roles in early adulthood, change roles in middle adulthood, and shed roles in late adulthood.