Figure 1 shows the mean SF-36 physical and mental component scores for each occupational grade plotted against age. Physical health deteriorated for all occupational groups at older ages, while mental health tended to improve with age. This change in SF-36 scores may be an ageing effect. Alternatively, the change may be caused by cohort effects (the oldest cohort member was born in 1930, the youngest in 1952) or period effects (such as retirement at any of the measurement phases).
Fig 1 Mean SF-36 physical component scores and mental component scores by age group: Whitehall II respondents from phases 3-7
Tables 1 and 2 show the mean SF-36 physical and mental component scores at each phase of data collection by age group, employment grade, retirement status, and sex. The physical component scores for men at phase 3 show that older men have poorer (lower) physical health. Comparing phase 3 with later phases, in each age group men from later phases also have poorer physical health.
Table 1 Mean SF-36 physical component score for Whitehall II participants, 1991-2004, with numbers in cohort shown in brackets
Table 2 Mean SF-36 mental component score for Whitehall II participants, 1991-2004, with numbers in cohort shown in brackets
We found similar patterns of age and period related declines in physical component scores among women, although the mean levels tended to be lower than in men. In contrast, older men and women have better (higher) mean mental component scores than younger men and women. Furthermore, at later periods there was a small increase in mental health scores for the older age groups (indicating a small improvement in mental health). There was a social gradient in physical health at phase 3, which tended to widen considerably among women, but less so among men. There was not much of a social gradient in mental health among men and women at phase 3. By phase 7, however, people from higher employment grades had better mental health than those from lower grades.
Table 3 shows the estimated coefficients from fitting growth curve models to the data in table 1. For the physical component score in model I, the constant term refers to the mean physical health (52.97) for the reference group: high grade, non-retired men aged 56 in 1991-3. A quadratic growth model (with age and age2 terms) was specified with a single year increase in age resulting in poorer physical health (a decrease of 0.03 from age 56 to 57). This negative effect of age on physical health was curtailed as respondents got older, indicated by the positive and significant effect of age2. A lower employment grade was associated with poorer physical health; at age 56, the gap between high and low grades was 1.60. Furthermore, the interaction between age and grade was negative; those in the middle grades aged 57 had a lower physical component score (−0.06) than when they were aged 56. There was a similar effect for the lowest grades (−0.05). This significant interaction term between age and grade indicates that health inequalities increase as this cohort got older. Over 20 years, the gap between high and low grades increased by 1 (20×−0.05). The trajectory of health decline for higher grades is not as steep as for those in middle and lower grades (fig 2).
Table 3 Estimates (standard errors) of physical and mental functioning and growth curve models fitted to Whitehall II men and women: full model
Fig 2 Trajectories of age related increase in SF-36 physical component and mental component scores by employment grade; Whitehall II phase 7 (estimated from model I, table 3)
The mean physical component score for an average high grade retired man aged 70 in 2002-4 can be calculated from model I in table 3 as 47.8. This is around the same mean physical component score for a low grade retired man from the same period aged 62. Thus, the average physical health of a 70 year old high grade man is similar to a low grade man around 8 years younger. There was a similar 8 year gap between high and low grades among women. In comparison, a high grade man aged 45 in 1991-2 had an estimated physical component score of 53.8, which is around the same as for a low grade man who was 4.5 years younger. So the age gap in physical health between employment grades widens from 4.5 to 8 years from mid-life to early old age.
Model II adjusts for confounders that could explain the widening health inequalities with age—namely, the interaction effects between grade, sex, and retirement (table 3). We included only significant interaction effects in the model. Neither sex nor retirement modified the effect of age, indicating that the trajectories of physical health did not differ by sex or retirement status. Women from lower grades reported poorer physical health than men from similar grades. Retired women also reported poorer physical health than retired men. The effect of retirement on physical health differed by grade, with retirement being associated with less of a decline in physical health for low grades compared to high grades. Despite the inclusion of these significant interaction terms, the interaction effect between grade and age did not change much compared with model I, indicating that the widening gap in health inequalities with age was not explained by the inclusion of these other interaction effects in the model.
For the mental component scores in model I (table 3), the constant refers to the mean mental health (52.22) for the reference group: high grade, non-retired men aged 56 in 1991-3. Ageing by one year was associated with better mental health (an increase of 0.16 in mental component score). A lower employment grade was associated with poorer mental health. Furthermore, the interaction between age and low grade was negative. Low grade men and women aged 57 years had a mental component score 0.07 lower than when they were 56 years old, indicating health inequalities increased with age. Figure 2 shows this for the latest period. At age 50, there was little social inequality in mental health, but a gap opened up as the cohort got older, with low grade older people not attaining the same high mental health as high grade older people.
In model II, neither sex nor retirement modified the effect of age, indicating that the trajectories of mental health did not differ by sex or retirement status. Women from the lowest grades reported better mental health than men from the same grade. Retirement for lower grades was associated with poorer mental health than in those in high grades. The interaction between low grade and age reduced significantly (from −0.07 to nearly zero), indicating that the widening gap in mental health inequalities with age was explained by the inclusion of the other interaction effects. The interaction between grade and retirement, in particular, explained most of the interaction between grade and age, suggesting that the widening gap in mental health with age is mainly because of the increase in mental component scores among the retired high grades.