I think Phael hit on an important point. It isn't any one thing, it's a combination of many things that probably make your voice come across as "young". It's diction, it's the phrases you choose, it's the cadence, it's the syntax. It is difficult to impossible to pin down what exactly makes the voice come off as young because, well, it's a mixture of things. But you can find specific places to focus on. You should probably pick apart a chapter sentence by sentence and analyze your choices: syntax, diction, the level of detail, the character's focus, character reactions, etc.
I think you may want to focus on syntax--in my experience, YA tends to use fragments in different ways than adult fiction. Also, if you use fewer complex sentences, it might read as "younger".
Once you pinpoint some things that make this voice seem younger, you might be able to consciously write "older" until it becomes second nature.
Some of it could be situational, of course, but it sounds to me like it's the words themselves and not the situation that are at issue. After all, lots of adult novels have teen POV characters.
This is a tough one, because voice is so non-specific. But if you want to write older and it isn't quite working, you have to figure out exactly what choices you're making that you could change.
Of course . . . maybe that age range is a sweet spot for you. There's no law that says you can't keep writing in that sweet spot.
I'm the same age as you (27), but I don't think I could go much younger than 16, and honestly I'm not sure I could go even as young as 18 for a contemporary teen (I write historical--different ball of wax). My sweet spot seems to be early twenties to mid-thirties.