1. Well, it's free, you can export your game to pc/mac/linux/iOS/Android/web, it's relatively easy to use (as a newcomer)
2. Well i guess if you really put an effort on it you can start at 12~15 it really depends on you, if you have prior experience making games maybe all you need is completing a tutorial before starting with your own game.
3. Because is easy, really complete, and let you export your game to pc/mac/linux/iOS/Android/web
4. Why not? I don't really know since i only do code.
5. I have only used 2 tutorials. This http://jessefreeman.com/learn-unity-2d/ and the infinite runner here https://unity3d.com/learn/tutorials/modules . Those are really good starting point, the only downside of the first is that is not free.
7. You can see for yourself here https://unity3d.com/showcase some of the games made with unity. I can tell you that making an infinite runner is very easy with unity.
8. It depends on what kind of game you want to make and how big you want it to be. It could be from 4 days to 2 years. It really depends on what you want to make and how much effort you put on it.
9. GooglePlay and pages like Kongregate as soon as you want/can since it is relatively easy and you can start to create a portfolio to show your work. Steam i'd say once you accomplish to have a decent fan base so they can help you greenlite your game.
10. Remember Flappy bird? They can go viral pretty easy with some luck, you can take advantage of current trends/memes, try to use leaderboards, people love to compete.
11. Start as soon as you can, it doesnt matter if you create just little games, thats great for a start. Create a lot of games, if you enjoy playing games try to also analyze them, what makes them fun?, what would you change?, how do they approach different things as the weapons, enemies, continues, everything; Even if you cannot recognize what everyone loves in a videogame (the truth is that there's nothing that will please everyone) you will find what YOU love in videogames and that way you'll find your style and feel for the games you'll create.
PS: You should really check out Extra Credit's youtube channel, great/fun source for game design theory.