To sign up, you should be between 13 and 17 years old. Your parent or guardian will be the account holder, allowing you to use it. Accounts for those under 13 will be available soon.
This is because the bank needs to make sure it's really you who's opening the account. If anything looks fishy with your account, they can use your phone number to reach you and check that everything is okay. They may also send you a code to confirm it's you when you log in or make a transaction. All of this helps keep your money safe and secure.
Right now, your US-based card can be used anywhere in the world where Visa cards are accepted. For example, if you are an American traveling internationally, you can use your Modak card anywhere!