Decreasing mother-to-child transmission is changing the population of children and adolescents with HIV. This project used recent epidemiological data to develop short-term projections of children and adolescents living with diagnosed HIV infection in New York State. A population simulation model was created to project prevalence of diagnosed HIV cases aged 0–19 years by age, sex, race/ethnicity and risk for years 2007–2014. Using 2006 data as the baseline population and 2001–2006 diagnosis and death data, annual diagnoses and deaths were calculated for each age/sex/race/risk category and known cases were ‘aged’ into the next year. The model produced annual estimates until 2014. The model predicts a decline in the number of persons aged 0–19 years living with diagnosed HIV in New York from 2810 in 2006 to 1431 in 2014, a net decrease of 49%. Living cases with paediatric risk continue to decrease. Cases aged 13–19 with non-paediatric risk increase slowly, leading to a shift in the risk composition of the population. The dominant effect seen in the model is the ageing out of perinatally infected children born before measures to prevent mother-to-child transmission were broadly implemented in the mid- to late 1990s. Changing trends in the young HIV-infected population should be considered in developing public health programmes for HIV prevention and care in New York State for the coming years.