The 18-year-old supervision question
Family Link
Family Link is built on account-level supervision through Google accounts. Per current Google policy, when the supervised account holder turns 13 they can choose to update their Google Account to a supervised teen account, but supervision continues. Teens aged 13 to 17 require parental approval to stop supervision through Google account settings. Supervision ends automatically when the supervised account holder turns 18. For parents wanting enforcement that continues past 18 regardless of the automatic age-out, Family Link's architecture does not support that case.
Anchor
Anchor is independent of Google's account supervision system. There is no age threshold at which Anchor's enforcement softens or unlocks. The four-layer moat applies to a 17-year-old the same as it applies to a 10-year-old, and the same at 18 or 19 if the parent keeps the pairing active. Whether to keep using Anchor as a child ages is a parental decision, not a software policy cutoff.