I want to be sure what are the requirements before any registration or payment. Some sites doesn't ask for my birth date.
It always depends on the regulation of your state. For some states it's 18, for others 19 and also the popular 21. In General it's required to be on at least 18 years. For Alabama it's 19 and 21 in Iowa, Kansas, and New Hampshire. However, this information is could be changed so you should contact the customers support of your website.
The important is to be an adult, responsible player and provide correct information. Any fake details will lead to closing of your account. It's not possible to use your pals details or credit card. When you sign up for this, you have to meet the requirements.