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.
It would be a surprise to you that children of this generation starts gambling activities from primary school,where they engage in playing PS2 games and bet on it.From that stage,they graduate to playing bets,since they already have the idea.If a particular child loves gambling,it doesn't matter the age,they can be 12,and will start gambling from that age till they grow big.