Apparently I was an exception, because even though I learned to gamble from early childhood, my grandfather always swore if he saw me playing "fool" with my friends. He was sure that cards are a bad toy for children and you can always come up with another, good and constructive game. And now that I've grown up, I understand it, and I think that even the age of 18 is too early for gambling.
I don't know about girls, but I think that guys under the age of 22 do not understand the laws of the world well, and do not have self-control. Therefore, it is necessary to allow gambling only from the age of 22.
Your grandfather knew better. And perhaps he is also a traditional one, hence he disapproved of you playing card games when you were still young because he also knew the possible consequences of gambling to a child. It can alter the way you think and how you perceive things. And I'm glad he made the right decision of reminding you to don't do stupid things that could cost you your future.
It's also good to know that you knew better today. It's really too early for a teenager to play gambling because a teen don't fully understand yet the dynamics of the world, the possible consequences, and doesn't even know how to decide on their own fully. Additionally, they don't even earn money for themselves yet. And I guess these are enough reasons why they shouldn't be allowed to gamble and be introduced to such.