To determine outcomes from early versus later age of transfer from pediatric to adult type 1 diabetes (T1D) services, and factors associated with adult service HbA1c. We used a longitudinal design from the Australasian Diabetes Data Network (ADDN) registry. Transferred youth (2013-2022) were compared by age group at last pediatric visit (<18 vs. ≥18 years). Multivariable generalized estimated equations (GEEs) were used to model factors associated with adult-service HbA1c. Explanatory variables included transfer age, sex, socioeconomic status, and mean HbA1c in the 36 months before transfer. There were 784 youth (n = 342 early and 442 late transferers; 49.7% male). The mean ± SD transfer age was 18.1 ± 1.4 years with median [interquartile range] 18.2 [5.5; 38.9] months of adult-service follow-up. The gap between the last pediatric and first adult visit was longer in early transferers (13.5 [5.5; 28.0] vs. 8.1 [3.7; 23.8] months; P < 0.001). At first adult visit, early transferers had higher HbA1c (9.3 ± 1.9 vs. 8.8 ± 1.9% [78.2 ± 20.6 vs. 72.9 ± 20.4 mmol/mol]; P = 0.01) and socioeconomic disadvantage (score 976.8 ± 101.7 vs. 992.8 ± 81.1; P = 0.03). In multivariable GEEs, higher adult-service HbA1c was associated with younger transfer age (β = -0.15; 95% CI -0.30 to -0.002; P = 0.047), socioeconomic disadvantage (β = 0.58; 95% CI 0.10-1.06; P = 0.02), and higher mean HbA1c in the 36 months before transfer (β = 0.06; 95% CI 0.04-0.07; P < 0.001). Early transfer from pediatric to adult T1D services was associated with higher HbA1c in real-world settings. There is a need for caution with early transfer. Findings should be interpreted in the context of other international models to understand optimal transfer timing.