Background Fast‐acting insulin aspart (faster aspart) is a novel formulation of insulin aspart (IAsp) ensuring ultrafast absorption and effect. Aim To compare the pharmacokinetics between faster aspart and IAsp, based on free or total IAsp measurement, and investigate the association between anti‐IAsp antibodies and faster aspart and IAsp pharmacological properties in children and adolescents with type 1 diabetes (T1D). Methods In a randomized, two‐period crossover trial, 12 children, 16 adolescents, and 15 adults (6‐11, 12‐17, and 18‐64 years) received 0.2 U/kg double‐blindsingle‐dose subcutaneous faster aspart or IAsp followed by a standardized liquid meal test. Results Across age groups, the pharmacokinetic profile was left‐shifted including greater early exposure for faster aspart vs IAsp irrespective of free or total IAsp assay. Onset of appearance occurred 2.4 to 5.0 minutes (free) or 1.8 to 3.0 minutes (total) earlier for faster aspart vs IAsp (P