There is no minimum age to apply, but the education/training requirements effectively preclude most people under the age of 18 from participating.
There is no maximum age to apply.
You can enter regardless of where you are living--even in the United States.
You do not have to speak or write English.
You do not need to have relatives living in the United States.
You do not need a particular amount of money or a have a job waiting in the US to apply.
You only need to qualify as an immigrant, which means that you cannot be a criminal, cannot be mentally disturbed, must be able to work or support yourself, and cannot be a threat to the US national interest.