The only "rights" that are age-dependant (as far as I can tell), are the right to vote (18 years), the right to enlist in the armed forces without parental approval (18 years), the right to sign a binding contract (18 years) and the right to get married without parental consent (18 years). 18-year-olds have the right to purchase certain categories of firearms as well (rifles, shotguns, muzzleloaders and ammunition). You must be 21 to legally consume alcoholic beverages or to purchase a handgun. I will not even touch the subject of drivers' licenses, since driving is not a right but a privilege. Carrying a knife or other weapon in schools and public buildings is prohibited for everyone, not just young people. Carrying a knife on the street? Like I said, if the knife is legal, anyone can carry it. If illegal, age will not play a factor. The only difference would be in the venue for prosecution (juvenile court vs. adult court).
the argument the an LEO can make is that it is a weapon, and legally you must be 18 to carry a "Weapon" given the whole "weapon" thing can be debated on the knife side and then argued on what really is a "weapon" but that is the legal justification that will be very hard to argue out of if the LEO wants to be a prick
-matt