In the US we need a consistent and meaningful "age of majority," reinforced with ritual.
We've got the 26th Amendment so let's stick with 18 for that "age of majority": legal adulthood. The privacy issues will be difficult, but at 18 every American should be issued The Card: passport, voter registration, registration for any conscription we still have (e.g., jury duty), I.D. for buying legal recreational drugs such as ethyl alcohol, and with one chip to be activated with licensing to drive and another to purchase fire arms — under whatever regulations are in place. And for each monthly cohort we can have a little ceremony with the oath or affirmation "to preserve, protect, and defend the Constitution of the United States" and a proclamation of this group as now adult citizens of the United States with all the rights, privileges, and obligations pertaining thereto — and then a party.
Some people are fully mature at 15; some people never grow up. And all sorts of stuff is happening in terms of neurological development that energetic lawyers and PR-savvy scientists have been talking about the last decade or so and will continue to talk about. Screw that. "Old enough to decide whether or not to obey an order to kill people, then old enough to perform any other adult function" (as a matter of policy; as mentioned "Individuals will vary, including individually, in different situations"). Wise societies depend on self-fulfilling prophecies, the psychological efficacy of ritual, and the social pressures of expectations.
Also we need classes for teens in civics and sex Ed (including sex-related ethics) and in using and not abusing drugs.