Hi Mark,
For Microsoft accounts, 18 is considered the adult age in most regions, which means accounts under 18 are treated as child accounts and remain subject to parental controls and consent requirements. At age 16, your son is still classified as a minor, even if his birthdate is correct.
This is why Microsoft continues to request parental consent codes when he signs in or accesses Microsoft services. To reduce or stop these prompts, sign in to family.microsoft.com with the parent/organizer account, select your son’s profile, and review the Privacy, App, and Content settings. You can disable “Ask a parent” prompts, turn off activity reporting, and allow unrestricted access where appropriate