For context I've learned Finnish, German, English, Russian, and Norwegian over the course of my life. But I am unsure if English should count because I am not fully confident in that one.
I can hold conversations in English though. Just not to degree levels.
I am also unsure if German should count because I was born in Germany.
But with that out of the way...
1. All of them at really different ages. I've known German and understood Russian since my childhood due to growing up around Russian adoptive parents.
I've learned Norwegian as a tween - teenager for my biological family, they live over there.
I've began studying Finnish in my late teens until adulthood just for lulz. Considered dropping it various times. Happy I never gave up in the end.
And English I started learning around 2 - 4 years ago.
2. German, self explanatory.
Russian, off and on for my entire life and learning from hearing others speak it constantly.
English, not fluent, skipping.
Finnish, 4 - 6 years.
Norwegian, about as long as a Norwegian toddler - child would need. So like also ~6 years right?
Never learn two languages at once, personally as a tip from me. I would've taken shorter had the other language not distacted me if that is important to you...
3. I think in German language in every case, and translate other languages to German in my mind.