In these discussions I always remember my English teacher in high school which was learning a new language, Spanish, at 56 (she was 3 years into it then). She believed religiously in immersion and that age is never a factor in language. Maybe she reached a zen state or something after learning 5 languages to fluency, or she straight up lied to me, but she was already instinctively using Spanish at that point.I can't think in Japanese as a native speaker would because I was not exposed to it when young enough.
I also started handling English later in my life than anyone I know, and it's as far apart from my native language as it is from Japanese, and I'm thinking primarily in English now. Age is definitely a factor, but I don't think it's as decisive as I hear everyone say, in the face of hard, time-sinked immersion.
Translation is a whole other beast though. I can't translate between English and my native language to save my life. I'm willing to believe that it exists in a complete separate part of the brain from fluency.