"Well, a younger woman is a type, but not necessarily a type for me. And what is a younger woman? I mean, I'm pretty old. Almost everyone is younger"

Jack Nicholson is implying that he is not interested in younger ladies, as he is already rather old. He is questioning what a more youthful woman is, as he is not exactly sure if he would even be thought about a practical alternative for somebody younger. He is implying that he is too old to be thought about a feasible choice for someone more youthful, as almost everyone is younger than him. He is also implying that he is not interested in more youthful ladies, as he is currently quite old. He is recommending that he is not the type of person that younger women would be interested in. He is also implying that he is not interested in pursuing a relationship with somebody more youthful, as he is currently quite old. He is suggesting that he is not the kind of person that younger women would have an interest in, and that he is not interested in pursuing a relationship with someone more youthful.