^Age is a lazy metric to measure people's knowledge about gambling. It's only a legal age that says you are now responsible for your action. The perfect gambling age (for casinos), of course, as soon as it's legal. But for the betterment of society, it should go deeper than just age. For example, you need to test your gambling knowledge, get the license, and show the license if you want to enter a casino.
Young adults seem more prone because they haven't got responsibilities yet. It's similar to single, young execs. But the latter already has job responsibility (quite busy). That said, any age bracket can have gambling problems, but the older you are, you got much more to do than just playing games.
That is the point of the question being held, what is the perfect age for gambling, looking at the lesser evils for the problems. As you said, no matter what age bracket, there will be a problem, but we are looking for right now is the age bracket with less problems and less issues in regards to gambling.
You said that young adults haven't got responsibilities yet, which I approve so they are probably not the best age for gambling, so I guess that beyond being young adult, where there is already responsibilities, yet they're still very eager and somehow younng.