A shift towards younger age at onset of diabetes in susceptible people has been suggested as a possible explanation for the increasing temporal trend in incidence of type 1 diabetes. We aimed to test this hypothesis by assessing trends in incidence rates in the period 1984–2004 in children and young adults in Northern Italy.

The study bases were: (1) children resident in the Province of Turin in the period 1984–2004 and in the remaining areas of the Piedmont Region in the period 1990–2004; and (2) young adults (15–29 years) resident in the Province of Turin in the period 1984–2003. Temporal trends in rates were analysed using Poisson regression models.

A total of 1,773 incident cases were identified. Overall incidence rates/100,000 person-years in the age groups 0–14 and 15–29 years were 11.3 (95% CI 10.7–12.0) and 7.1 (95% CI 6.6–7.7), respectively, with sex differences among young adults only (incidence rate ratio [IRR] in males vs females 1.41 [95% CI 1.20–1.64]). Average annual increases in incidence rates were similar in children and young adults at 3.3% (95% CI 2.5–4.1). Compared with the period 1984–89, in 2000–2004 a 60% higher risk was found in both age 0–14 years (IRR 1.60, 95% CI 1.31–1.95) and 15–29 years (IRR 1.57, 95% CI 1.26–1.96) groups. The Poisson modelling showed no interaction between calendar period and age at onset.

Incidence of type 1 diabetes in Northern Italy is increasing over time in both children and young adults, not supporting the hypothesis of a shift towards younger age as the main explanation for the increasing temporal trend in children.