While this creates staffing responsibility, it's far less trouble than having a censorious mayor breathing down librarians' necks, trashing them in the press, and unplugging the library's computers. Older kids should be given nearly unlimited access to the Net. This is not a favor we grant the young, but an inherent right. They have the right to access new technology, and to choose their own culture. By high school, young people should be able to explore the Net and the Web in almost any legal way they wish.
Supervision can be much more relaxed and occasional.
Library staffers or volunteers can walk by computer terminals to make sure jarring sexual images aren't present. If hard-core pornography is downloaded and staffers or other kids spot and report it, kids can be warned. But at this age, kids should know how to navigate the Net safely. If they don't, it's an issue between them and their parents. The library will have done everything possible by offering instruction and supervision from the earliest possible age.