This trial is conducted in Europe. The aim of this trial is to investigate the exposure of NN5401 (insulin degludec/insulin aspart) in young adults and elderly subjects with type 1 diabetes mellitus.

Inclusion Criteria:

• Male or female aged 18-35 years (both inclusive) (young group) or at least 65 years (geriatric group)
• Type 1 diabetes mellitus (as diagnosed clinically) for at least 12 months
• Body mass index 18.0-28.0 kg/m^2 (both inclusive)
• HbA1c (glycosylated haemoglobin A1c) maximum 10.0% by central laboratory analysis

Exclusion Criteria:

• Subject who has donated any blood or plasma in the past month or more than 500 mL within 3 months prior to screening
• Smoker (defined as a subject who is smoking more than 5 cigarettes or the equivalent per day)
• Not able or willing to refrain from smoking and use of nicotine gum or transdermal nicotine patches during the inpatient period