Gambling is addictive, keep that in mind. If you are fifteen years old, where will you get your money to gamble? If it is from your parents, then there's nothing to argue here. They are more likely not working eight hours a day to support you with such activity. And likewise if you have a job for some reason, because you should be using it on things that would benefit you for a long run.

Just to site a reference and for you to understand it more, here's an example study of why there is a restriction.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2945873/

Actually there are more which validates the claim of not allowing minors to gamble. The main thing is "addiction". Anything that would be too much is not advisable. Regulation won't be enough because there are things wherein once you are exposed to it, avoidance becomes a problem.