Children aged -15 years are never suitable for gambling because it can damage their behavior later in life.
It depends on their mindset. At the age of 15, I doubt they will have an interest on doing gambling.
But if they really did, more on it was because of their curiosity. I'm sure at that age they already know the difference between what's the good thing to do and not. If we try to force and convince them to ignore gambling the more they want to know about it.
Gambling is all over the internet these days but as parents, we should know how to protect our kids from it no matter where they are. It might sound hard since we don't know what they're doing when they're outside but if we'll explain to them the risks of gambling and the effects of it, they would know how to avoid it. It's how we raise our children.