Why do you need to be 18 to use Apple cash?

Hi

In order to use Apple Pay Cash, you must currently be a resident of the US (and at least 18 years old):

Send, receive, and request money with Apple Pay - Apple Support

If you are also asked to verify your age and identity, then it is because the provider of the service (a regulated financial institution) requires that information in order to comply with federal law.

Identity verification for Apple Pay Cash - Apple Support

Because the terms and conditions require it, as detailed here:

https://applepaycash.greendot.com/termsconditions/

If you still have questions:

"If you have any questions regarding this Agreement or the Services, please contact us at (877) 233-8552."

You are not talking to Apple here - this is a user-based technical support community.