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 will 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.