GoKawiilApple's Trade In page is where you check the trade-in value of various devices. For iPhones, the oldest model you can trade in and still get credit for is the iPhone 8, which came out in 2017. Upon picking a phone from the list, you'll need to indicate if the phone is in Good or Damaged condition. Your phone is still considered as Good even with minor scratches or dents. In this shape, Apple offers a mere $35 for the iPhone 8.
If you choose the latter option, you'll be asked several more questions about what is specifically wrong with the phone. But even if you indicate that a damaged iPhone is functional and its body/display are in good shape, you may not get anything for an older model.
2017's lineup also included the iPhone 8 Plus and then-premium iPhone X. Those with an iPhone 8 Plus can get $40 of trade-in credit if the device is in good shape, or $10 if it has limited damage. An iPhone X in good shape warrants $60, with no value for a damaged model. Keep in mind that trade-in amounts are estimates — you might get less than quoted if you didn't describe the condition accurately.
Apple's trade-in list goes all the way back to the iPhone 5, but you won't get any credit for these obsolete phones. If your device is too damaged or old to accept, the company will only recycle it.