I think it's a good idea to have it registered for at least 3 years... they SE's can see this, so it's a wise choice. I don't think it's going to make a big difference in rankings, but it's not going to hurt so why wouldn't you?
It depends on how long your domain has been alive for and not how long you are planning to keep it for.
The age of a domain is important. It's just like sending your kid to school. Would you trust a newbie teacher more than a teach who has been teaching for let's say 10 years? This is the same case with google. The older domain has priority.
I guess this thread needs to be locked?? everyone knows the answer and all are typing the same stuff again and again
whilst it is true that older sites placement is usually higher, google does not consider time span of the registration period as a ranking factor. The higher seo is due to visits and backlinks built up over time.