"1) When did parkour start in America?:"
Sketchy at best, but as people have said give or take 20 years.
"2) I assume it's pretty obvious, but is there any girl here?:"
I think Muse satisfied this answer.
"3) Where do you often go to practice? Is there any commonplace that the groups meet?"
I have never specifically gone to a certain place to train parkour, since it was always impromptu and I was unaware that I was participating in a worldwide art form without realizing it. But I plan on training specifically for parkour and free running in local parks, my apartment complex, and any place I happen to come across that looks appealing.
"4) From my noobie perspective, parkour culture is quite similar to skateboard's one: it develops on streets, mostly young men... Is that right?"
Parkour has developed in similar and also in quite different ways than skateboarding. similar in that word of mouth, or more often than not, sight of eye is our best ambassadors. Skateboard as an extreme sport was centered around tricks and stunts, whereas stunts and tricks are merely a by-product of parkour. Thus Skateboarding was launched into a competition atmosphere very quickly. As far as their cultures go, you could say that Parkour is only a culture, and develops just as any other culture develops, through flow of information. So in that way yes, you could compare the cultures. But the reasoning behind the two are very different. Skateboarding, from what I understand, is more about accomplishment, and parkour is about efficiency. Thus I do not believe parkour will follow the same pattern of growth that skateboarding has gone through. /rant off
"5) About you guys, what is your occupation? (students, martial art, body training, related fields, I guess?) Because parkour is not easy. So maybe only people of some specific fields are able to do this?"
My age is nineteen, student, no background in any acrobatic discipline, just have been blessed with an athletic body. And the age range here, as people have stated is quite varied. True that excellent traceurs often have roots in acrobatically challenging disciplines such as martial arts and gymnastics. However Parkour is not about flips, twists, and tricks, it's about efficiently flowing through your environment, like electricity, you find the path of least resistance. True that people enjoy watching flashy moves like a Kong to Gainer, but these are not the core nor the concentration of parkour. That being said, it is a sport that requires you to be in good shape, but anyone can attain this with enough drive and power of will, it just takes effort. This is the key element that most people lack and therefore you see more people that have already developed discipline and training (i.e. gymnasts and martial artists) embracing this art form. But anyone can learn and enjoy Parkour. So no it is not only for people of certain physical dispositions.
"6) What do you like about parkour?":
I like the physical benefits of having a practical everyday use for being in great shape, not just working out to get a ripped bod like many people these days. I like the versatility of the entire discipline as it caters to being an extremely useful skill whether you are jogging in the morning, or fleeing from a pursuer. I like the feeling of confidence and contentment after a tricked is successfully completed, I like the philosophy of Parkour being a lifestyle, not just a sport, I like the people that participate, I like the unity that it brings through different cultures, creeds, and backgrounds. I like the name, I like the pain of hard work, I like that it makes me feel more alive, I like that it challenges myself and those around me. I like that it is something I have always done spontaneously without ever knowing I was doing it.
"7)The last one, if anyone has a special moment, a moment that made you choose parkour, or a moment that changed the way you think about it?"
My second story childhood room had a roof extending out from it. I was an insomniac as a kid, and so at night after everyone went to sleep, I'd open my window, go out on the roof, and jump off into a roll, then I would go to our woods (We lived out in the country) The trees in those woods were mostly Yaupon, they grew very close together and were not thick at all, I would climb a tree at one end of the ten acres we lived on, and attempt to climb and jump from tree to tree without ever touching the ground until I reached the road which was about 500 feet away. This was my first regular forays into parkour, and I loved doing this at night while the world slept. Brought me a lot of peace. As far as a specific moment, before I did this, I would just sit on the roof. The defining moment for me, would have to have been deciding to jump off that roof for the first time. I was 9.
I hope at least a little bit of those ramblings will help you on your research.
Good luck, and good choice for a topic!