That's really cool. The reason I ask is that is that I had a friend who went to live in Japan, became fluent and echoed the same response as you.
When I first came to Australia I could barely speak a word of English. I was turning 4 years old at the time, my default language was French. After reaching 6-7 years old I did dream in French on occasion, but 95% of the time it was in English. I think I stopped thinking in French the majority of the time after around 8-10 years old. It's interesting, the French thoughts and dreams only arise when attached to certain people in my life.
Maybe if you learn a language when you're out of childhood, you're also learning to actively think in that language because you hadn't had to previously do it. I think when you're a child under the age of 7 your brain development is rampant and you just pick up things like there's no tomorrow without having to think too much about it.
Yup, agreeing with you on the linguistics learnt at a young age thing. Even if fluency isn't reached, those roots, and the ear for other languages, stay firmly wired in your brain. I grew up in a tri-lingual family (not even counting different dialects), and while I annoyingly never became fluent in more than English after starting school, I have the ear for languages. It's why I find it really easy to watch and listen to stuff in say, Japanese, and pick a few bits up here and there. My problem now is though I get bored when trying to go back and learn properly the other languages, as I find the basics boring and too easy. The bits though I do speak I think in that language. I've seen similar with friends in academia who've got kids and have lived in different countries. The kids, like I did, mix languages, and will even jump between langauges in a single sentence, with some words one langauge, and the rest in another.
As for picking up accents not my own, for a vampire larp I used to play in, I put on a really cheesy 'Transylvanian' accent - yeah, I was playing a character who thought she was one of the Draculesti, and sometimes she'd put on a 'posh' English accent to try and fit in. The really anoying thing was, after the game once, I couldn't shake this awful, really hammy, fake Transylvanian accent! I had to really force myself to speak with my normal, very neutral accent. It was funny mind you. XD