It’s unlikely to cause any kind of large black market. In the UK, energy drinks have been restricted to 16+ for a few years now. People don’t mind off brand vapes or cigarettes (for some reason), but people absolutely care about the brand of energy drink they are buying. Kids especially, I imagine most only buy them to look cool, and if no one knows you are drinking a “cool” energy drink then why bother?
Did you know that, in the UK, every year, there is enough lithium in all the disposable vapes that are used, to make 1200 electric car batteries?