Tanning beds should be banned for people under the age of 18 because of the intense skin damage they can cause. The usage of tanning beds greatly increases the risk of developing skin cancer and melanoma. It would be just like with cigarettes - they cause cancer, so the government has banned children from being able to buy or smoke them because they are harmful. People should have freedom to make their own choices, but by preventing people to use tanning beds until they are old enough to make more informed decisions for themselves, it might prevent people from increasing their risk of preventable disease.
Individuals under the age of 18 are not considered adults in our society. As such, they cannot vote, they cannot get tattoos, they cannot buy cigarettes, they cannot get legally married-- at least not until they are deemed old enough to make an informed decision. I think the same should be true for tanning beds, a practice that has well-known detrimental effects.
We are obsessed with regulating responsible behavior. We are also responsible for labeling objects with icons and symbols and warning labels. Responsible behavior cannot be legislated; it can only be encouraged. You can fine and arrest people until they are green in the face but it won't stop people from doing dumb things. It would be better for culture and society to encourage responsible behavior than to deem things hazardous, dangerous and unlawful.
I would argue that as long as a person is under the age of 18 years old, their legal guardian should be the one who decides if their child uses a tanning bed (in moderation, of course). If an employee or someone else suspects the use is excessive, it could be reported. Otherwise, let parents parent their children as they wish. We don't stop parents from feeding their kids all the toxins from the candy and snack food aisles leading to obesity; how is a tanning bed any different?