No reason why it would or should be. Caffeine is fine to use up to threshold of around 400 mg/day. For comparison a 12 can of regular Pepsi contains 38 mg. Energy drinks typically have a significantly higher caffeine content, and in addition often have additives, like taurine, which can have effects on the cardiovascular, muscular and nervous systems. Add in the fact that people who consume energy drinks tend to excessively consume them, much like soda drinkers do, and it’s not necessarily a good combo even for adults. That can be worse on adolescent and teen bodies, which are still developing. And to make matters worse, energy drink brands tend to advertise heavily towards sectors that have a high youth audience – such as videos games, concerts/music and sports events (especially “extreme” sports). Small wonder than some retailers, as a matter of policy, just won’t sell these products to customers under a certain age.