Even non dissociative people have parts of self that can feel like a five year old or a 16 year old or whatever. My t once specifically gave me an example of being in the grocery store and having a younger voice in her head saying things like "Oooh, I want those!!" and then her more adult self replying (in her head) "Well, no, that's not healthy, I'm not going to buy that" (with more back and forth going on). My T doesn't have and never has had a dissociative disorder. But she does have parts of self, like everyone does.
As to why you're only noticing this now... maybe you're only paying attention to your own inner experience now. I really only started paying attention to my body this year. Realized I've been living in pain for 20 years from a condition I never thought to mention to the dr before, because although I was in pain, I was acclimatized to it - I wasn't "listening", I wasn't paying attention. Maybe you're only noticing this now because you're in a space where you can pay attention to your self, to focus on your self.