four.
By the time Ron was six, he knew enough to know that the Weasleys didn't have a lot of money. They didn't have enough money to get Bill, Charlie, or Percy the kind of school things they wanted. They complained a lot.
By the time Ron was eight, he knew enough to know that other wizarding families had a lot more money than they did. He resented them. He especially hated the Malfoys. His dad hated the Malfoys. He would come home from work at least a few times a week grumbling about them, and how Lucius Malfoy, whoever that was, got off with it. Whatever that meant.
By the time Ron was nine, he could make the connection that rich and wealthy people were greedy, mean and unliked. The Malfoys were very rich. Nobody liked them. Harry Potter, The Boy Who Lived, had nothing. Everybody like him. And he was Ron's age. Why didn't everybody like Ron?
Ron decided that he should be rich one day.