I've had several people ask me how they should refer to me when talking about me the past, so I've spent some time thinking about it myself, and my preference is that people use my current name and pronouns. From my perspective, coming out and transitioning didn't change my identity; it revealed it.
When I was little, I was a girl who was actively missing out on a lot of the 'girl stuff' that the other girls I knew got to experience, and that often made me feel sad and frustrated and left out, even if I didn't fully understand why at the time. I don't personally feel that using my current name and pronouns to refer to my younger self is 'rewriting' my past or being deceptive, nor does it really erase any of that pain - I'm just speaking from my own perspective about how I've experienced life, regardless of what other people might have assumed about me or how they treated me.
Just for the sake of minimizing any potential issues, if I'm talking to someone that I'm not out to and who I worry might not be a good ally, I'll be vague and generic about my past with things like "when I was little", "when I was younger", etc. If I'm out to everyone involved in the conversation or it's otherwise a 'safe' environment though, then I have no problem referring to my younger self as a girl and I just ask them to follow the same rule that I do as best they can, and to just be generic about it or direct people to me if they're unsure.
Using a person's current name and pronouns when discussing their past seems to be the most common preference among the trans* people I've interacted with. If it's someone I know, I'll ask how they want me to refer to them and in what contexts, and I use whatever they prefer; otherwise using a person's current name and pronouns seems like the most polite approach.