The way they do it is that they convert all of the grades everybody has into an overall percentage (for instance a paper out of 90 in which somebody got 45 marks right would be 50%) and then they decided where the boundaries should fall from there. So one year it might be 86% for a 7, the next year it might be 84%. Being honest they very very rarely vary by more than one or two percent in any direction, and it all depends on how hard the exam has been. If the percentages were set, that wouldn't allow for the fact that all exams are different and of different difficulty levels.
There are also two different timezones in which exams are sat (otherwise people would be able to cheat off whichever timezone had to do the exams first) and because the papers are different between the timezones, they have two different sets of grade boundaries - one for timezone one and one for timezone two.
No idea where you'd find the average, but not all that many people actually get 7s. Seeing as it's out of 7, you could have my guess that the average mark is probably 3, 4 or 5

Not very helpful, I know.
Grade boundaries differ between courses (dependent on how hard a subject is....) so no averages probably wouldn't be the same for all courses in terms of the percentage required to reach the average grade.
Google percentages of people who get all the different marks, perhaps? If you're interested. Honestly it doesn't really matter in the IB as it'll have no real bearing on your own grade or ability - everybody sits the same exam, after all, so if you're better than people you'll stay better than them whether the grade boundaries are high or low thanks to the way it's distributed
You don't have much time in the IB. It's not a course for anybody who wants a decent social life, unless you can absorb information like a sponge and do your work extremely efficiently and to a tight time schedule! Most people have social lives mediochre to poor. Being honest, it's not a course which everybody in the world could do, it's designed for a specific sort of person (an academic person

). If you're smart in school and aren't totally averse to learning and working, you'll cope fine. From a personal point of view, however, if you have a national qualification which you've nothing against, I'd do whatever is normal for your country in preference to IB. IB doesn't offer you anything extra (except for work!!)