Background The main objective of this study was to compare future glycemic control in children diagnosed with type 1 diabetes mellitus (T1DM) at toddler age and preschool/school age. In addition, we aimed to examine risk factors known to be associated with future glycated hemoglobin A1c (HbA1c) levels in children diagnosed with T1DM. Methods This is a retrospective cohort study of 85 patients diagnosed with T1DM at toddler age (group 1; 0-2.9 years; n = 36) or preschool/school age (group 2; 5-6.9 years; n = 49) who were followed up at the University Children's Hospital in Zurich for at least 10 consecutive years or until the age of 15 years. Results The mean HbA1c level in the first year after diagnosis had a highly predictive value about glycemic control in the following 6 years. In addition, a longer duration of T1DM was associated with higher HbA1c values. HbA1c values did not differ significantly within 11 years after diagnosis between children in the two age groups. Neither was a difference found when comparing the two groups in respect to their chronological age, although a trend was noted (p = 0.09). This trend is very likely due to a longer duration of diabetes in group 1. Conclusions HbA1c level in the first year predicts glycemic control for the next 6 years and deterioration of HbA1c values can be noted with longer duration of T1DM. Moreover, our study demonstrated similar future glycemic control in patients diagnosed with T1DM at toddler age and preschool/school age.