Can I buy an iPhone if I'm under 18 years old?

Purchases are not age restricted in any way. As long as you can pay for it, nobody can stop you form buying the phone in store directly regardless of age. They should not even ask for your age at all.

Family Sharing is not involved when buying physical items from a physical store.

If you already have a card, then there should be no issue. There are Credit cards for people under 18 . Apple will not question age or card usage in any way.

You can pay anyway you like.

Hi xyetwas!

Apple Family Sharing refers to sharing services (such as iCloud Storage or Apple Arcade). It does not refer to any hardware purchase or cellular service plan.

While many retailers may be willing to sell you an iPhone, you will need to check with the Cellular Service Provider's procedures to find out what age restriction (if any) they may have to open a new account/cellular line. If you are going to transfer your Cellular service, you will want to check with your Cellular Service Provider before purchasing the iPhone.

Hope this helps!

You cannot get your own credit card if under 18, you can be an authorized user:

https://wallethub.com/answers/cc/how-do-you-get-a-credit-card-under-18-2140664118/

Best to check with the credit card company because we are all just other users and do not work for Apple or the bank.

Almost any way. Apple no longer accepts check or travelers checks.