The baseline characteristics of the study population by 5 year strata of age and diabetes duration are shown in Tables 1 and 2, respectively. Overall, the mean (±SD) age of the cohort was 65.8 ± 6.4 years and the age at diagnosis was 57.8 ± 8.7 years (Table 1), with 21%, 18%, 22% and 39% of patients reporting their age at diagnosis as ≤50, >50–55, >55–60 and >60 years, respectively. The mean (±SD) diabetes duration was 7.9 ± 6.4 years, with 43%, 27%, 18% and 12% of patients reporting a diabetes duration of ≤5, >5–10, >10–15 and >15 years, respectively (Table 2). The mean diabetes duration of the 5-year age strata increased linearly from the youngest to the oldest but only varied by 1.5 years across the age groups (p for trend <0.0001, Table 1). The mean age of the groups by 5-year diabetes duration strata was very similar, except for the group with the longest diabetes duration (>15 years) who were on average only 1.2–1.5 years older than the groups with shorter diabetes duration (≤15 years) (Table 2). By contrast, the mean age at diagnosis of the group with the longest diabetes duration (>15 years) was significantly lower than that of the groups with shorter diabetes duration (p for trend <0.0001, Table 2). Age, age at diagnosis and diabetes duration were all significantly correlated (p < 0.0001), with the strongest correlations evident for age and age at diagnosis (Spearman correlation coefficient 0.69), and diabetes duration and age at diagnosis (Spearman correlation coefficient 0.65).
Table 1 Baseline characteristics by 5 year age strata
Table 2 Baseline characteristics by strata of diabetes duration
Macrovascular events and death
After base adjustment for randomised treatments and further adjustment for baseline HbA1c level, age, age at diagnosis and diabetes duration were all associated with risks of macrovascular events and all-cause death (all p < 0.001; Tables 3 and 4, Fig. 1a–f). For each 5 year increase in age (or age at diagnosis), the multiple adjusted risks of macrovascular events and all-cause death were increased by 33% and 56%, respectively (Tables 3 and 4). For each 5 year increase in duration of diabetes, the multiple adjusted risks of macrovascular events and all-cause death were increased by 13% and 15%, respectively, when accounting for age (Table 3), or increased by 49% and 78%, respectively, when accounting for age at diagnosis (Table 4). Compared with the effects of a 1 SD increase in diabetes duration on macrovascular events and all-cause death, which allows some direct comparison of relative effects, the effects of a 1 SD increase in age and age at diagnosis were markedly greater (Electronic Supplementary Material [ESM] Tables 1 and 2).
Table 3 Hazard ratios of adverse outcomes associated with a 5 year increase in age or duration of diabetes
Table 4 Hazard ratios of adverse outcomes associated with a 5 year increase in age at first diagnosis of diabetes or duration of diabetes
When the components of the macrovascular outcome (fatal and non-fatal myocardial infarction or fatal and non-fatal stroke) were examined separately, the associations were consistent (ESM Table 3).
When the base models were adjusted for multiple potential confounding covariates (sex, systolic blood pressure, BMI, lipids, smoking status, prior microvascular disease or macrovascular disease, renal function and urine albumin:creatinine ratio) the associations were slightly attenuated but remained significant (ESM Table 4). When these models were further adjusted for mean HbA1c level before the index event, the associations remained evident, except for that between duration of diabetes and macrovascular events, which became marginally non-significant (ESM Table 4).
No interaction was observed between the effects of age or age at diagnosis and diabetes duration on the risks of macrovascular events and all-cause death (all p for interaction >0.098, Tables 3 and 4). Across all strata, older age, older age at diagnosis and longer diabetes duration increased the risks of macrovascular events and all-cause death (Figs 2a, b and 3a, b).
When participants with a history of macrovascular disease at baseline were excluded from analyses, the results were unchanged (ESM Table 5).
Microvascular events
After base adjustment for randomised treatments, age at diagnosis and diabetes duration were associated with risk of microvascular events (p < 0.0001) but age was not (p = 0.2889) (Tables 3 and 4, Fig. 1g–i). After further adjustment for baseline HbA1c level, diabetes duration continued to be associated with risk of microvascular events (p < 0.0001, Table 4) but age at diagnosis was not (p = 0.2882, Table 4). For each 5 year increase in diabetes duration, the multiple adjusted risk of microvascular events was increased by 28% (Table 3).
When the components of the microvascular outcome (new or worsening retinopathy and new or worsening nephropathy) were examined separately, the associations were consistent except that age was inversely associated with new or worsening retinopathy, such that after base and further adjustment for baseline HbA1c, older age was significantly associated with lower risk of retinopathy (ESM Table 3).
An interaction was observed between the effects of age or age at diagnosis and diabetes duration on the risk of microvascular events (both p for interaction <0.05). Overall, for every 5 year increase in age or age at diagnosis, the effect of a 5 year increase in diabetes duration was reduced, with greater risks of microvascular events observed in those who were younger rather than older for the same duration of diabetes (Tables 3 and 4). However, when the associations were examined in those with short duration of diabetes (0–5 years), there was a weak positive relationship between age or age at diagnosis and microvascular events, whereas in those with long duration of diabetes (>10 years) there was a clear negative relationship between age or age at diagnosis and microvascular events (Figs 2c, 3c).
When participants with a history of microvascular disease at baseline where excluded from analyses, the results were unchanged (ESM Table 5).
Competing risks
In analyses taking account of all-cause death as a potential competing risk, the results for macrovascular and microvascular events (ESM Fig. 1) were almost identical to those from the primary analysis (Fig. 1).