It always seems like a bad idea to attempt a technological solution to a legal/social problem. Surely the age of people using services is a purely a social issue - similar to not allowing alcohol to be sold to underage drinkers. This kind of law would be analogous to making a law that alcohol cannot be physically consumed by underage drinkers.
[edit] The social argument for banning or restricting substances is also limited when it comes into too great a conflict with the social argument for tolerating individual choice. To even put the freedom to speak privately on the same plane as the freedom to take drugs is to abandon reason itself, not to mention hundreds of years of hard-won liberties and enlightenment. Anti-drug regimes are merely authoritarian. Anti-privacy regimes are totalitarian. There is a world of difference between policing action and policing thought.