I used to think this. I was in my late 20s but still felt like a teenager in my head.

At some point in the last few years, after I crossed into my early 30s, I realized that wasn't true anymore. I don't feel like a teenager, I just feel like a 20 something now. Which is still incorrect but there's definitely been a shift.

Maybe it stopped because when I'm around teenagers, I realize how much distance I feel from them. Not in a "kids these days" way, just in a general sense. A feeling like "...oh...I'm not like this anymore. I remember being like this, I still kind of am, but I haven't really been like this in a while." The juxtaposition is so evident that my unconscious self-perception can't maintain the denial.

I certainly don't feel my age, but my "internal age" (so to speak) has progressed a bit. I guess it's a sliding scale.