This paper challenges the focus on age 18 as an exclusionary point in law for migrant young people, particularly unaccompanied migrants, with insecure legal status. Initially meant to provide a protective category of “childhood” in law, focus on age 18 creates a sharp transition point in law for young people. This chronological concept of age does not match up with the reality of lives of many young people who step into adulthood without being able to live in a self-supporting manner. Law recognises the constraints and provides some respite for British national children who are in care; however, non-UK migrant and/or asylum-seeking young people in this situation are immediately at risk of losing their liberty. We suggest that non-British migrant young people aged 18–21 should be treated as a youth category in a manner similar to that used for British young people in care.