If there's a way to write "immaturely", I don't think that being over 35 will automatically help you to avoid it. =/ To be honest, I'm not sure what that means in this context.
Based on the examples given (Twilight, Hunger Games), I'm guessing that it refers to plots focused on young love and the strife that comes with learning how to form and maintain a romantic relationship in unusually stressful/life-threatening situations. And thus, someone with a "mature" writing style would not do that.
But that seems a little...restrictive. Like, there's nothing inherently immature or underdeveloped about writing about such things; I'm sure it's possible to do it with maturity and critical thinking, and honestly one could argue that those books already do (I've never read the Hunger Games books, but from what I've heard about them they sound fairly mature and insightful to me).
Anyway, personally I don't think there's any point in trying to find a 'grown up voice', especially as someone who writes a lot of sci-fi and fantasy (genres that are automatically labeled "for kids" no matter what you write, unless it includes lots of gratuitous sex and violence).
For me, the appeal of the YA novels I used to read was the risk, the adventure, and the perspective of someone relatively ignorant or innocent, stepping into a world they didn't quite understand yet...and none of those things are exclusive to YA. So I just continue to use them, and if my writing "matures" it'll be because I'm creating more complex characters, not because I'm leaving behind perspectives/plotlines that just happen to be associated with what teenagers like.