Age 18 - this info is stated in the first section of the User Agreement.
Below is a snippet:
This user agreement is a contract between you and PayPal, Inc. that governs your use of your PayPal account and the PayPal services. It applies to U.S. PayPal accounts only. For an individual to open a U.S. PayPal account and use the PayPal services, you must be a resident of the United States or one of its territories and at least 18 years old, or the age of majority in your state of residence. For a business to open a U.S. PayPal account and use the PayPal services, the business must be organized in, operating in, or resident of, the United States or one of its territories.
By opening and using a PayPal account, you agree to comply with all of the terms and conditions in this user agreement, so please read all of the terms and conditions carefully. The terms include an agreement to resolve disputes by arbitration on an individual basis. You also agree to comply with the following additional policies and each of the other agreements on the Legal Agreements page that apply to you:
