"Look," she said, "you have to swear not to tell Richard," although she realized, even as she said it, that she'd never quite understood this prohibition, "but Eliza has leukemia. It's really terrible."
To her surprise, Walter laughed. "That doesn't seem likely."
"Well, it's true," she said. "Whether or not it seems likely to you."
"OK. And is she still doing heroin?"
A fact that she'd seldom paid attention to before – that he was two years older than she was – suddenly made its presence felt. (2.2.394-397)
Why does this conversation cause Patty to realize that Walter is older than her? Maybe because it shows her naïveté – that she is too easily trusting. and life experience (that is, experiences like these) will make her more wary of believing people, and bring her to expect the worst of people (specifically, to expect people to lie).