400
"Even in your twenties you know how old you are. I'm twenty-three, you say, or maybe twenty-seven. But then in your thirties something strange starts to happen. It's a mere hiccup at first, an instant of hesitation. How old are you? Oh, I'm – you start confidently, but then you stop. You were going to say thirty-three, but you're not. You're thirty-five. And then you're bothered, because you wonder if this is the beginning of the end. It is, of course, but it's decades before you admit it."
Who is Jacob talking about how he doesn't know how old he is. He's just old. He refers to not knowing your age as "the beginning of the end"; in others words, once you have to think about how old you really are, you're aging too quickly to keep up with.