
##### Multi-generations

This family image includes an elderly woman and her young-adult daughters and granddaughters from the Hmong ethnic group in Laos. Grandmother and daughters are adults, but they are obviously far apart in age. What ages define the beginning and end of adulthood?

Adulthood is the life stage between adolescence and death, but assigning exact ages to the beginning and end of adulthood is not easy. The event that marks the end of adulthood (death) is clear cut, but the age at which it occurs varies considerably. The beginning of adulthood is equally difficult to identify exactly. A person may be physically mature and a biological adult by age 16 or so, but not defined as an adult by law until older ages. For example, in the U.S., you cannot join the armed forces or vote until age 18, and you cannot take on many legal and financial responsibilities until age 21.

Adulthood is generally the longest stage of life, potentially lasting for up to 80 years, or even longer. The man in Figure $$\PageIndex{2}$$, for example, is pictured celebrating his 110$$^{\text{th}}$$ birthday, so he has already experienced over nine decades of adulthood! Although most physical growth and maturation are finished by the time adulthood starts, many changes occur during these decades of life. As a result, adults of different ages may be quite different from one another. Therefore, it makes sense to divide the long period of adulthood into stages, such as the stages of early adulthood, middle adulthood, and old age.

Early adulthood coincides more or less with the 20s and early 30s. During early adulthood, people generally form intimate relationships, both in friendship and in love. Many people become engaged or marry during this time. Often, they are completing their education and becoming established in a career.

Strength and physical performance typically reach their peak between 20 and 35 years of age (see Figure $$\PageIndex{3}$$). All sexes reach their peak fertility in the 20s, and for females, fertility starts declining in the 30s. Health problems in young adults tend to be relatively minor. Cancer is rare in this stage of adulthood, but there are a few exceptions, notably testicular cancer, cervical cancer, and Hodgkin’s lymphoma. The most common causes of death in young adulthood are homicides, car crashes, and suicides.

Middle adulthood lasts from roughly the mid-30s to the mid-60s. Note that this age range is longer than the stage of life commonly called “middle age,” which is usually considered to range from about 45 to 65. During middle adulthood, many people raise a family and strive to attain career goals. Community involvement is also common in this life stage.

Middle adulthood is the stage when most people start showing physical signs of aging, such as wrinkles and gray hair, like the gray-haired woman in Figure $$\PageIndex{4}$$. Typically, vision, strength, aerobic performance, maximal heart rate, and reaction time also start to decline during middle adulthood, although there is great individual variation in the ages at which these changes occur.

Fertility continues to decline until menopause occurs, typically around age 52, after which they are is no longer fertile. Fertility in male sexes starts to decline after age 40. Most middle adults start to diminish in height — especially females who have osteoporosis, which is common after menopause. Up to 1 cm of height per decade may be lost. Some people experience a small degree of cognitive loss during middle adulthood, but this usually goes unnoticed, because life experiences and accumulating wisdom generally compensate for the loss. Middle adulthood is also the time when many people develop chronic diseases such as type 2 diabetes, cardiovascular disease, or cancer. These diseases are the chief causes of death in middle adulthood.

### Old Age

Old age begins in the mid-60s and lasts until the end of life. Most people over 65 years of age have retired from work, freeing up their time for hobbies, grandchildren, and other interests. On the other hand, retirement may lead to less social contact and loneliness. Many elderly adults may also be exposed to prejudicial treatment because of their age (ageism). For these and other reasons, depression is very common during old age, and people over the age of 65 have the highest rate of suicide.

#### Physical and Cognitive Changes in Old Age

Physical declines that started in middle age continue during old age. Declines generally occur in stamina, strength, reflex times, and the senses. As they grow older, most people become increasingly frail, with loss of muscle mass and lessened mobility. However, there are many exceptions. Some people remain fit and active in old age. The man in Figure $$\PageIndex{5}$$ is an outstanding example.

Many people suffer from multiple chronic health conditions in old age. The immune system becomes less efficient, increasing the risk of serious illnesses, such as cancer and pneumonia. As people age, the number of brain cells also decreases. Nearly half of those over the age of 85 years exhibit at least mild cognitive impairment. Diseases such as Alzheimer’s disease that cause serious and permanent losses of mental function also become more common.

#### Age at Death

Old age ends at death, but when is death likely to occur? The average age at death is reflected in the statistical measure known as life expectancy. Life expectancy is defined as the average time an individual is expected to live. It is based on the year of their birth and their current age and gender. In the United States in 2015, life expectancy at birth was 77 years for males and 82 years for females.

Life expectancy is just an average, and many people outlive the life expectancy value for their year of birth and gender. In fact, by the year 2050, a projected half a million Americans will be at least 100 years old, thanks to a large number of baby boomers and advances in health care. Is there an upper limit on old age? In 2016, scientists identified the maximum human lifespan as 115 years on average, with an absolute upper limit of 125 years. These numbers may increase if scientists learn how to slow down aging. This requires understanding the causes of aging.

## Causes of Aging

Why do we decline in so many ways as we age? Why is there an upper limit on the human lifespan? The causes of aging (and ultimately death) are not known for certain, but a number of factors have been proposed. These factors fall into two general categories: programmed factors and damage-related factors.

### Programmed Factors

Programmed factors follow a biological timetable and maybe a continuation of the timetable that regulates childhood growth and development. An example of a programmed factor is the shortening of telomeres. Telomeres are regions of repetitive nucleotide sequences at the ends of chromosomes (Figure $$\PageIndex{6}$$). They may normally serve a variety of functions, such as protecting chromosomes from fusion with neighboring chromosomes. Telomeres become shorter each time a cell divides, and when telomeres become too short, the cell stops dividing and dies.

Damage-related factors include internal and external assaults on the organism that produce cumulative damage to DNA or cells. Many damage-related factors have been proposed, including the following:

• Exposure to environmental mutagens: Mutagens may damage DNA, and DNA damage can prevent cells from dividing. There are several checkpoints in the cell cycle where cell division is halted if DNA damage is detected.
• Accumulation of waste products in cells: Waste products may interfere with normal cellular metabolism. The amount of waste might reach a level at which cells can no longer function.
• excessive amounts of highly reactive chemicals called free radicals (for example, OH-): Free radicals can damage DNA and cells, and contribute to diseases such as cancer and cardiovascular disease. These diseases are major causes of death in the latter decades of life. Some causes of excess free radicals include exposure to environmental pollutants, drinking alcohol, eating trans fats, and smoking tobacco.
##### Feature: Reliable Sources

Popular media outlets sell books, diets, and programs that promote the calorie restriction theory of anti-aging. They make money by telling people how and why to eat less in order to live longer. Is this just hype or wishful thinking? Or are there real longevity benefits to calorie restriction? Research reliable sources to find answers to these questions.

## Review

2. Why is it difficult to give exact ages for the beginning and end of adulthood?
3. List the stages of adulthood.
4. Describe the stage of early adulthood.
5. What is the age range of people in middle adulthood, and what are some of the changes that typically occur during this life stage?
6. Define old age, and describe this stage of life.
7. What does life expectancy measure? Identify two factors that influence life expectancy. What was the life expectancy of Americans born in 2015?
8. What is the maximum human lifespan?
9. Discuss possible causes of aging.
10. A 40-year-old person is typically considered to be
1. in old age
2. middle-aged
4. B and C
11. Compare the chief causes of death between early adulthood and middle adulthood.
12. Why do you think scientists are studying how to lengthen telomeres?
13. Free radicals and mutagens both cause damage to what structures?
14. True or False: Once a person reaches adulthood, their height stays constant.
15. True or False: Life expectancy is generally lower in females than in males.

## Explore More

Dr. David Sinclair is a researcher who studies the causes of aging, and how we may be able to inhibit the process. Watch the talk below to learn more about this exciting research.

1. Black H'mong family by Bob Tubbs, public domain via Wikimedia Commons
2. Shelby Harris by Rikeshia Davidson, public domain via Wikimedia Commons
3. Serena Williams by Wikigo, CC BY 3.0 via Wikimedia Commons
4. Ellen Langer by Robert Scoble CC BY 2.0 via Wikimedia Commons
5. Fauja Singh by Mithrandirthewise, public domain via Wikimedia Commons
6. Telomere caps by U.S. Department of Energy Human Genome Program, public domain via Wikimedia Commons
7. Male borderline underweight by Fredrik public domain via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

This page titled 23.8: Adulthood is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.