If you are talking about a ban on a server, then it's however long the server owner made it.
When you buy the account, it lasts forever.
You should be able tto download it as soon as you've paid for it.
There is no age limit, as long as you buy an account you have one.
Anyone can register a free Minecraft account on the Minecraft website.
Anyone can make a Minecraft account at Minecraft.net
A full minecraft account costs $26.95
You must register your account to play Minecraft, however, anyone can download Minecraft.
What do you mean by this? If you simply mean have minecraft on more then one computer then of course you can. As long as you have a valid account you can download minecraft onto any computer.
You can play Minecraft from as many computers as you want as long as you log in on each one from your account.
It's not the account that costs in Minecraft; it's the download. The cost to download Minecraft is $19.99.
You have to get an account on the real minecraft website.
Buying Minecraft for the PC costs $26.95 as of September 20th, 2014. It is a one-time payment, and as long as you remember your account information, you will never need to buy another account.