To assess the effects of age, birth cohort, and period on comorbidity rates as well as project their future trends over the next 25 years.

Population-based retrospective observational study.

Record linkage from the population-based healthcare utilisation database of Lombardy, Italy, between 2004 and 2023.

All beneficiaries of the Italian National Health Service (NHS) aged 50–85 years residing in Lombardy. Data were separately analysed for each year from 2004 to 2023, with thus the availability of 20 study populations.

Comorbidities were traced via the medical services provided by the NHS, and the overall quantification was obtained by the Multisource Comorbidity Score, which was developed and validated for the Italian population. The temporal analysis of the 20 yearly temporal comorbidity rates was obtained by the Age-Cohort-Period models. The comorbidities prevalence trends were forecasted from 2025 to 2050.

From 2004 to 2023, the prevalence of comorbidities declined from 46% to 40% in men and from 47% to 42% in women. An increase in prevalence between the ages of 50 and 85 years was observed for both women (from 33% to 63%) and men (from 29% to 67%). A declining prevalence was observed among cohorts born from 1922 to 1970 for both women (by 33%) and men (by 50%). A continued decline in the absolute number and prevalence rate of comorbidities is expected for both women and men until 2050.

The decline in ageing-related comorbidity prevalence over time may persist up to 2050. Improved medical care and public health initiatives benefiting individuals born in more recent years may counterbalance the expected trend of increasing comorbidity prevalence due to population ageing.