The old trope is that when you start going in for psychoanalysis, they start by asking you about your parents. I guess if you’re going to start talking about traditional games, you should probably start with Dungeons and Dragons.
Not that Dungeons and Dragons is not necessarily the first traditional game we all play; far from it. I was playing Jr. Monopoly and Battleship long before I picked up my first D20. But it kind of marks the threshold, at least for a lot of people. The point where games stop being the thing you do when your bored out of your skull on a rainy day and starts being the thing you love.