The age at which one is considered "older" or "senior" is always evolving and is influenced by culture and societal life expectancy. In England in 1875, old age was defined as 50 years or older, as stated in the Friendly Societies Act [88]. Today, most developed countries in the world use the chronologic marker of 65 years as a definition of old age; in some cases, the age of 62 years is used as a chronologic marker because, in the United States, one could receive social security benefits starting at this age [87]. However, Mohanty notes that using the criterion of age at retirement or when one becomes eligible to receive retirement benefits is not universal, given the fact that there are so many in the world who live in areas in which there are no formal definitions of retirement [88]. Using these chronologic markers to define old age is arbitrary, but they can be useful when studying the group as a whole [2]. Older adults can be further divided into various segments: the young-old, defined as 65 to 74 years of age; middle-old, defined as 75 to 84 years of age; and the oldest-old, defined as those who are 85 years of age and older [3]. In 2021, 32.5 million Americans were 65 to 74 years of age, 16.5 million were 75 to 84 years of age, and 6.7 million were 85 years of age and older [31]. In 2010, there were 53,364 persons 100 years of age and older in the United States [94]. By 2020, there were 92,000 U.S. centenarians [32]. Worldwide, in 2015, there were more than 500,000 centenarians, and it is estimated by 2050, there will be 3.7 million, with the greatest growth in China [90]. In the United States, it is projected that there will be 9.7 centenarians per 10,000 people by 2050 [90].

Biologic and physiologic changes are part of aging. Although it is not known why these changes occur, biologic theories of physiologic aging include [3,7,86,95,121,140,143,91,105]:

The losses in the physical arena for older adults can be numerous, which may then compound and/or have implications in social and psychological arenas. Studies have shown that brain tissues atrophy due to natural cell degeneration, with the volume of the brain decreasing by 15% or more between adolescence and old age [122]. Crews notes that the health status of older persons with vision and hearing loss is poorer compared with those without vision or hearing loss [10].

In industrialized countries, it is estimated that 10% to 15% of human immunodeficiency virus (HIV) infections occur in adults 50 years of age and older, and approximately one of every nine new HIV diagnoses in the United States occur in those 50 years of age or older [17,18,126]. In 2019, there were 3,100 new cases of HIV among adults 55 years of age and older [225]. Between 2015 and 2019, the rate of new HIV diagnoses remained stable among those 55 years of age and older [226]. This prevalence may be higher in developing countries [18]. In 2016, there were an estimated 6,812 newly diagnosed cases of HIV among adults 50 years of age and older, accounting for 17% of total diagnoses in the United States. Of these, 841 diagnoses occurred in persons 65 years of age and older. In 2015, approximately 47% of all individuals with living with HIV were 50 years of age and older, and 2,749 people 55 years of age and older died from HIV disease [97]. However, older adults are not generally considered an at-risk group. This has led to a lack of targeted education and screening among older adults. For example, only 32% of state departments of public health websites contained information about HIV/AIDS in older adults [167].

Erik Erikson, a prominent developmental theorist, had a more optimistic view of aging, focusing on the positive ways of overcoming the various crises one encounters throughout life [25,26]. Erikson postulated eight stages of psychosocial development. Each stage provides the individual with a choice of two alternatives to consider and accept; one is an opportunity for growth, while the other results in unhappiness. In late adulthood, individuals confront the challenge of integrity versus despair. During this stage, individuals reflect on their lives, and determine if they have lived a life of purpose. If so, the individual will feel contentment, having attained integrity [26].

In cases of terminal illness, grief counseling should begin before death occurs, with a focus on life meaning and contributions [37]. Awareness and understanding of the mediators of grief responses can assist in recognizing individuals who may be at increased risk for adapting poorly to the loss. These mediators are [38]:

In 2018 in the United States, 56% of those 65 years and older required long-term services [213]. Institutions providing long-term care to older individuals often provide a variety of services, including personal, social, and medical services. Key factors that predict older adults entering a nursing home include [111]:

There are several screening tools available for assessing older adults with problem drinking. The Drug Abuse Screening Test (DAST) is a 28-item questionnaire consisting of yes or no responses. The Short Michigan Alcohol Screening Test (SMAST) is a 13-item questionnaire with a similar response format. These instruments are commonly used but may not be appropriate for the older adult population. However, a longer version of the SMAST, the Michigan Alcohol Screening Test-Geriatric (MAST-G) was specifically developed in order to accurately assess alcohol abuse and dependence in older adults [46]. The MAST-G consists of 24 items, which may limit its incorporation into regular screening procedures. As opposed to the standard MAST, this version focuses more on drinking in response to grief and changes in drinking patterns over time.

Increasing or intensifying social networks can involve various approaches. Cattan and White identified three key aspects of interventions for older adults that support active engagement: group activities that provide support (e.g., support for bereavement), interventions targeted to a specific subgroup (e.g., widowers), and activities or interventions emphasizing older adults' control [70]. Several interventions for older adults that focus on decreasing social isolation have been identified and may be classified into four general categories [71]. The first type of intervention to address isolation is one-on-one telephone support services, whereby a counselor checks in with older adults periodically. Second, teleconferencing, whereby a group of older individuals are brought together via a phone conference, has been found to be a cost-effective and useful intervention. Third, face-to-face support groups were found to be beneficial in reducing social isolation, particular groups that last for at least five months [71]. Lastly, with increasing Internet accessibility, e-mails and Internet support groups can also be beneficial for older individuals. Older adults who have access to and use computers have more social support and are less likely to experience loneliness [217]. A study exploring the use of videoconferencing with frail older adults in Australia found that the patients preferred videoconferencing with a pain specialist over a face-to-face consultation [116]. A systematic review found that social connectedness and support were increased in older adults who used Internet technologies, although the effects were short term [187]. As discussed, loneliness is often intertwined with other variables, such as shyness or limited resources. Therefore, interventions should not simply focus on providing venues for older adults to meet; interventions and programs that focus on people's expectations about friendships and relationships are equally important [157].

The issues of an older adult's mental capacity and self-determination come into play with informed consent, particularly if cognitive impairments are present. Informed consent involves three dimensions: the communication of the information, the opportunity to ask questions, and the process of making a decision [162]. As stated, an older adult's self-determination should be promoted. It must involve not only the communication of the information but also giving older adults the opportunity to ask questions about their care, the intervention, and/or services provided to them and empowering them to make the decision. However, there may be times when an older adult cannot give informed consent. One way to assess if the older adult understands the intervention is to have him/her reiterate what the intervention entails [6]. However, there are a few options when an older adult's capacity is compromised. First, a surrogate caregiver could provide consent. Second, double informed consent could occur, whereby the surrogate caregiver gives informed consent and the older adult gives assent. Finally, it is possible to obtain early informed consent from the older person via legally binding documentation of wishes prior to any cognitive impairment [6]. Practitioners may consider the following questions when assessing capacity [266]: