Workplace injury and illness is a serious concern in today's work environment. The estimated annual direct and indirect cost of workplace injury and illness is approximately $250 billion, a total greater than the annual cost of cancer.1 Most of these costs result from injuries/illnesses that require time away from work or work restriction.1 Unsurprisingly, workplace injuries/illnesses that require a greater amount of time out of work or limitation while at work tend to be more costly.2–5 As such, there is interest in better understanding the factors related to the length of disability and how to best manage the work-disability process. Age has been consistently shown to relate to the experience of workplace injury/illness and the length of disability. Research has found that the length of disability increases with age, as does the likelihood of experiencing work-disability recurrence. In addition, the chances of never achieving a return to work (RTW) increase with age.6–13 There are several potential reasons for the relationship between age and RTW outcomes that include biological changes associated with aging, which might delay recovery after injury/illness, older workers might sustain more serious injuries/illnesses, and there is a higher incidence of comorbidities with age that might further complicate recovery.14,15 In addition, it has been suggested that older workers might have reduced access to rehabilitation services, older workers might also be less satisfied with their rehabilitation services, employers might not encourage older workers to RTW, and older workers might be in industries and occupations with fewer opportunities for accommodation that might facilitate RTW.8,15 Despite the research on the relationship between age and the length of disability, research also has demonstrated that the length of disability decreases with increasing tenure.6,9,16–18 The relationship between tenure and the length of disability might be due to several factors including the increase in work experience at a given company that can help workers to better navigate the RTW system at an organization, workers with longer lengths of tenure might also have greater organization attachment and more positive supervisor interactions both of which can make the RTW process easier, and more accommodations might be available to workers with higher tenure. The reverse directions of the relationships between age and the length of disability and tenure and the length of disability are somewhat surprising as age and tenure have traditionally been very closely related.19 As such, it might be expected that a positive relationship between age and the length of disability would translate to a positive relationship between tenure and the length of disability; however, this is not the case. These findings raise questions including the following: Are age and tenure independently related to the length of disability? Is one of the relationships stronger than the other? Is there nonlinearity in these relationships? and Do age and tenure interact in their relationship with length of disability? Few studies have systematically examined models involving age, tenure, and the length of disability. Several studies have, however, included age and tenure in predictive models either as covariates or as variables included in larger multivariate models. Findings indicate that both explain unique variance in work-disability outcomes.6,9,20,21 In the majority of previous work, age and tenure have been included in models with only linear terms or in some cases as categorical variables, despite speculation about possible nonlinearity in the relationships.15,22 There has been only one previous study that has considered nonlinearity in both relationships. In this study, the age/work-disability relationship was found to be linear; however, an inverted u-shaped relationship was found between tenure and certified sickness absence. Previous research has not yet examined nonlinearity in the relationships with disability resulting from work-related injury/illness specifically. In addition, research has not examined whether the relationship with the length of disability is stronger for age than tenure, or vice versa. Regarding the possible interaction between age and tenure, again there is limited research. A few studies have examined the interaction between age and tenure with the risk of experiencing a workplace injury/illness. One study found that in younger railway workers, the risk of injury increased with tenure.23 Another study found that in older health care workers, the risk of injury was the lowest for new hires.24 Other research has focused on the interaction in relation to the length of disability. In a study of workers in the fruit and vegetable packing industry, the number of days off work following an accident was found to be similar across tenure levels for middle-aged workers, whereas for older workers the number of days off decreased with tenure.25 A final study found different patterns for the interaction between age and tenure with the length of certified sickness absence among two groups of workers. For homecare workers with high tenure, age was positively associated with the length of certified absence, whereas there was no association with age for homecare workers with low tenure. In contrast, for residential care workers, the positive relationship between age and certified absence was stronger for low- and medium-tenure workers than for high-tenure workers.15 These studies focused on single industries, raising questions about the generalizability of the findings. This is an especially important time to examine relationships among age, tenure, and the length of disability. The demographic makeup of the United States workforce has been changing drastically over the last several decades. In 1990, roughly 12% of the workforce was comprised of workers age 55 and older. That number is expected to grow to over 25% by 2020.26 This has heightened concerns for employers about the potential impact of the aging workforce in regards to the cost and length of disability for older employees.27 At the same time that the workforce is aging, the typical career trajectory is also changing. Traditionally, individuals entered the workforce after school and then remained with a single employer for their entire careers. As a result, as age increased, tenure on the job also increased; however, research indicates that this trend might be changing.28 Individuals now change organizations, and sometimes careers, several times throughout their working years.29,30 This has led to shorter average lengths of tenure, especially for men at older ages. For example, the median tenure for male workers aged 55 to 64 years fell from 15.3 years in 1983 to 9.5 years in 2006.31 Based on previous research, it is unclear whether and how age and tenure might interact in their relationship with the length of disability; however, there are a few potential directions this interaction might take. First, work-related experience generally increases with tenure. As such, this experience might serve as a protective factor against the age-related changes that result in an increase in the length of disability as age increases. If this were the case, the length of disability might be shorter for older workers with higher tenure compared with those with lower tenure. On the contrary, workers who have been with their employer longer might feel increased job security, which could result in individuals taking a longer time to RTW as they do not feel the need to rush back to work to keep their jobs. Accordingly, older workers with higher tenure might have longer disability durations than those with lower tenure. The current study aimed to address these possibilities. The changing demographic makeup of the US workforce and the changing nature of career trajectories make this an important time to conduct research aimed at gaining an understanding of how age and tenure might interact to influence work disability. The goal of the present study is to explore the relationship between age and tenure with the length of disability following a work-related injury/illness to better understand the complex interplay among these factors.