"Look guys, I'm not 30 yet!" Gyllenhaal cracked at Saturday's New Yorker Festival in Manhattan. "Can you cut it with the 30 comments?"
Deflecting – humorously – any questions about leaving his 20s behind, the Love and Other Drugs star may feel wistful, but he also says he is comfortable with the person he has become.
"I feel more like myself. I feel like now, it's just the beginning," he says. "I guess you get to a point at 30 or somewhere and you say, 'What I am, is what I am and that's what I'm going to be and that's all good. I don't need to be anything else.' And that's kind of how I feel."
One major influence is big sister Maggie Gyllenhaal, 32, who initially sparked Jake's love of acting. Growing up, the two did a lot of performing when their parents held dinner parties.
"Some of my earliest memories are of my sister coming back from doing a rehearsal for some musical during elementary school," he says. "I would watch her, and I realized that she got attention that way. I joined her and we would put on shows together and she would always boss me around."
Typically, Maggie would pull the big-sister card. Recalls Jake, "She would put on a stage performance of Cats, and I would be a lone, poor cat sitting in the corner while my sister performed. I never got to be the star. I did everything wrong according to her."
Now that both are Oscar-nominated headliners, what would it take for them team on the big screen again, as they did in 2001's Donnie Darko?
"I love her a lot," Jake tells PEOPLE, but "we don't go around looking for something to work on together. I would rather spend time with my niece, [Maggie's daughter Ramona, 4] and spend time as brother and sister."
Not that the door is completely closed. "It would take something that I love so much … that I would need her to be a part of, and vice versa," he says.