Prone to cavities. Children tend to be more prone to decay than adults. That being said some adults do get the Fluoride treatment.
Several Reasons. First, people are more prone to getting dental disease early on and stabilize later in life (usually). It is more effective to get the Fluoride incorporated into the enamel when the teeth erupt - before they get decay. Second, insurance usually only pays for Fluoride for kids. I haven't had any adults in 20 years want to pay for it if it's not covered even when it could really be beneficial.