Insulin degludec (IDeg) is a basal insulin with an ultra-long pharmacokinetic profile in adults that at steady-state produces remarkably flat and stable insulin levels; however, no studies have yet reported on the pharmacokinetic properties of IDeg in subjects younger than 18 years of age. This was a single-centre, randomised, single-dose, double-blind, two-period crossover trial conducted in children (6–11 years), adolescents (12–17 years), and adults (18–65 years) with type 1 diabetes. Subjects received a single subcutaneous dose of 0.4 U/kg IDeg or insulin glargine (IGlar), respectively, on two separate dosing visits, with pharmacokinetic blood sampling up to 72-h postdose. A total of 37 subjects (12 children, 13 adolescents, and 12 adults) completed the trial. Total exposure of IDeg after a single dose (AUCIDeg,0-∞,SD) was higher in children compared to adults [estimated ratio children/adults 1.48 (95% confidence interval, CI: 0.98; 2.24)] and in adolescents compared to adults [estimated ratio adolescents/adults 1.33 (95% CI: 1.08; 1.64)]; however, the difference was only statistically significant for the latter comparison. No statistically significant difference in maximum concentration of IDeg (Cmax,IDeg,SD) was observed. Estimated ratios for Cmax,IDeg,SD were (children/adults) 1.20 (95% CI: 0.90; 1.60) and (adolescents/adults) 1.23 (95% CI: 1.00; 1.51). Simulated mean steady state pharmacokinetic profiles supported a flat and stable IDeg exposure across a 24-h dosing interval. IDeg was detectable in serum for at least 72 h (end of blood sampling period) in all subjects following single dose. In conclusion, the ultra-long pharmacokinetic properties of IDeg observed in adults are preserved in children and adolescents with type 1 diabetes.