Adolescence and emerging childhood forms an increasing proportion of the lifespan of urbanized individuals. Glycaemic control worsens during adolescence; physiology and psychology contribute. A1C levels peak around 9% (75 mmol/mol) before declining from late teens onwards. However, unchanging glycaemia (tracking) is common. Glycaemia has generally improved in the past 10–15 years, but significant differences between and within countries persist. Microvascular complications are prevalent at this stage, but have probably also decreased with time. During this important period, the stage can be set for premature macrovascular disease (early onset hypertension, arterial stiffening, dyslipidaemia, and smoking). Exercise reduces the risk of microvascular complications. Smoking is as common in young Type 1 patients than in the general population. Efforts at smoking cessation need reinforcing. Glycaemic control during university does not improve. Transition from paediatric to adult diabetes services is often unsatisfactory; clinics should implement simple procedures focusing on accessibility, flexibility, and improved communications.