That's correct, inselpeter.
Of course I can't pin an exact age when it started happening, it was more of a gradual process as you can imagine. The more I understood and practiced English, the more it took over me. At some point (by the age of 17-18, or alternatively 2-3 years ago), it surpassed my Hebrew (as I never took into Hebrew studies to answer your second question). Nowadays, in fact, I can read English books more easily than Hebraic ones!
Is it difficult to think in English and then sort of automatically translate to Hebrew while speaking.
Oftentimes yes. It's actually very inhibitive, because you can't say what is straight off your mind, and it definitely makes communication less fluent (and you might even say cumbersome). In a way I'm my own translator - imagine that!
Do you find yourself mixing the two languages, either in thinking or in speaking.
In thinking I differentiate Hebrew and English, and, even though rarely, I still happen to think in Hebrew. But I hope to suppress this nasty old habit completely one day.
In speaking-- only if I talk to those who I know understand English, then I often mix the languages.
Edit: A lot of times I find myself translating what people say from Hebrew to English, just for fun and for practice.