1. Introduction

1.1. Subjective Age of Older Adults
Subjective age refers to the degree to which people feel younger or older than their chronological age [1]. Like chronological age, subjective age contributes to a variety of developmental outcomes [2]. People who feel younger than their chronological age are usually better off than those who feel their actual age or older [3]. For instance, the outcomes of one of the only meta-analyses that has investigated the longitudinal effect of subjective age on future health and longevity among adults (average age 57–85 years) revealed that feeling younger is connected with improved physical health and longevity [4]. Furthermore, a correlation has been found between younger subjective age and important developmental processes, such as enhanced subjective wellbeing [5], better cognition test performance [6], and having fewer depressive symptoms [7]. Research has found that feeling younger than one’s chronological age is associated with higher levels of subjective wellbeing [5], greater life satisfaction [8,9], and more positive affect [5,10]. Younger subjective age is also associated with having a sense of meaning in life, higher levels of optimism, and more successful aging [8]. Similarly, feeling younger correlates with a decreased likelihood of experiencing a major episode of depression [11] and reduced symptoms of depression [12].
In another meta-analysis, women reported a younger subjective age compared to that reported by men [13]. Later studies have provided more evidence that being a woman is correlated with younger subjective age [5,14,15]; however, others have not found this correlation [6,16,17,18]. The impact of loneliness on subjective age is not clear; one study found that a decrease in loneliness resulted in a decrease in subjective age, but, changes in objective social indicators did not predict changes in subjective age [19].

1.2. Loneliness in Older Adults
The definition of loneliness is the gap between real and wished for social relationships [20,21]. Similar to subjective age, loneliness is a subjective concept and not an objective social manifestation [20]. Although loneliness can be connected with objective aspects of the social network including the number and frequency of actual social contacts, it is not synonymous with these aspects and it still represents the qualitative elements of relationships [22].
With a few exceptions, the majority of past research has emphasized the role of loneliness as a strong predictor of morbidity and mortality [23,24]. A considerable body of research has pointed out that a high level of loneliness is a major risk factor for cardiovascular diseases [25], disability [26], poor sleep hygiene [27], impaired cognition [28], and impaired physical functioning [22]. These negative effects of loneliness might explain the possible association between feelings of loneliness and subjective age, and in contrast, the negative correlation between size of friendship network and self-perception of age (for example, the finding that women who felt themselves younger than their actual age had larger friendship networks [29]. Moreover, a recent study suggested a plausible causal model of loneliness leading to morbidity and mortality, and found evidence of mediation by subjective health, depressive symptoms, and functional limitations [30].
Although objective indicators of social relationships also predict health and well-being [31,32], these are generally thought to exert a somewhat smaller effect in comparison to loneliness. However, there is general consent that loneliness increases with age among older adults [33]. This is not surprising given the many objective losses that take place in advanced age [33]. Objective losses include, for example, retirement because it is often associated with the narrowing of one’s social network or the death of a spouse, siblings, and close friends, which also result in reduced social contact [34,35].

1.3. Mediating Factors
In addition to the possible direct correlation between loneliness and subjective age, during a crisis like the Coronavirus disease (COVID-19) pandemic, feelings of loneliness can have far-reaching implications for the life of older adults, which could shed light on some of the effects of feeling lonely on subjective age during a crisis. Two specific mediators are likely to be at play—depressive symptoms and malnutrition.

1.3.1. Depressive Symptoms
According to the 2001 World Health Organization (WHO) Global Burden of Disease Study, depression is a serious public health problem among individuals, families, and societies throughout the world. The WHO estimated that depression was the fourth leading contributor to the global burden of disease in 2000, as measured by disability-adjusted life years [36]. Although depression rates are generally lower among older adults (5.4%) compared to middle-aged (9.8%) and younger (7.4%) adults, the rate among seniors in the United States has continued to rise in recent decades [37,38,39]. In addition, the reported rates of depression in the older population of the US may be underestimated. This is because depressive symptoms can be masked as physical complaints or initially appear to be cognitive impairments in this age group, moreover, the stigma of mental illness may inhibit depressed older adults from seeking treatment.
Depression is one of the negative health outcomes linked to loneliness as well as disability, and cognitive decline [40,41,42,43]. For example, a study in Ohio retirement communities found that older individuals who reported feeling lonely had significantly higher depression scores [44]. However, the experiences and consequences of loneliness may vary greatly. Moreover, because subjective age reflects self-perception, it is also related to many psychological factors among older adults. Research has shown that people who feel younger than their chronological age tend to be mentally healthy and have fewer psychological problems [11,12]. In addition, younger subjective age has been associated with less stress [1], fewer depressive symptoms [45,46], and strong mastery beliefs [45,47].

1.3.2. Malnutrition
Malnutrition is defined as a state in which a deficiency, excess, or imbalance of energy, protein, and other nutrients causes measurable adverse effects on tissue and body form (body shape, size, and composition), function, and clinical outcomes [48]. It is more prevalent as age increases [49,50,51]. The etiology of malnutrition is multifactorial; adverse physiological, psychological, and social causes of malnutrition in older adults are consistently reported in the literature [52]. Aging is accompanied by physiological changes that can negatively impact nutritional status, for example, sensory impairment may result in reduced appetite and poor oral health, and dental problems can lead to difficulty chewing, inflammation, and a monotonous, poor-quality diet. Progressive loss of vision and hearing may also limit mobility and affect the ability to shop for food and prepare meals [53,54].
In addition to loneliness and depression, other psychosocial and social changes characteristic of older adults, such as cognitive impairment, heavy use of medication, periods of lengthy hospitalization, retirement from paid work, bereavement, and increasing frailty can also contribute to poor nutritional status [54,55]. These factors affect the ability of older adults to meet dietary needs or to digest, absorb, utilize, or excrete nutrients that are ingested, leading to reduced energy intake and lean body mass. This, in turn, may result in a reduced metabolic rate, a corresponding decline in total energy expenditure, and potentially to malnutrition [56,57,58]. Thus, malnutrition, like other unhealthy outcomes of old age, may also be associated with subjective age [45].