1) It's free and offers the promise of being able to create almost any kind of game.
This fits in well with the grandiose dreams many beginning would-be game designers have.
2) Whenever you're old enough to understand what you're doing, more or less. Define 'effectively'. You can create something simple without knowing a whole lot about the program or its features if you limit yourself to what you can achieve with what you actually know.
3) In comparison to what? It's free, which is a big plus for a lot of people, but so are some others, at least non-commercially. Take something simple like a hammer. You have claw hammers, rip hammers, framing hammers, ball peen hammers, roofing hammers, sheetrock hammers, ... Each is great for the task it is designed for. You pick the tool based on your need - if Unity will do the job you need done, then it's a good candidate for you.
4) If you're going to create your own art and audio it's not a recommendation, it's a necessity. Nothing does everything.
5) I'm sure there are dozens. Some of them are probably even good ;) I don't use Unity so I can't answer this one specifically.
6) Yep - using it, and putting your creations up for harsh but constructive, honest critique by other game designers.
7) You could make any kind of game you want with it. Different games have different requirements and skillsets needed other than knowing how to use Unity. RPG's probably have the most diverse requirements, while match-3 games require a lot less. This goes back to Unity being a tool. Knowing how to use a hammer doesn't make you a carpenter. Knowing how to use Unity doesn't make you a game designer.
8) Most games never finish their development cycle, lying on the heap of abandoned projects and dreams that were just plain too large. If you're talking about completed games, it can be anywhere from months (maybe even weeks) to years. Again, it depends on the game - and that has nothing to do with Unity, per se.
9) Not until you've got a large fanbase of people playing your games, asking for more, and possibly buying them direct from you (proving that people are willing to exchange hard earned dollars for some of your entertainment).
10) How did pet rocks make their 'inventor' a millionaire? There's no accounting for taste and fads.
11) Think of the game you want to build. Now write it down in a folder and put it away somewhere, you're not going to build that for a long while. Take what you think is a small, achievable game. Now cut that back even more. Build something simple you know you can finish. Your first 'games' can be a single level platformer where you 'win' if you reach the end, or if you have to do an RPG, one with a very small town, a forest, and a very small dungeon. If you can put the dungeon entrance inside the town, that's even better. Tic-tac-toe, pong, Space Invaders are all great early projects.
Finishing something is an important skill to learn early.