That is genuinely shocking to hear. You have to be like 3+ sd above the mean. Do you know how many charge cycles you have? I recall that most batteries need replacing after 300-500 cycles.

My iPhone 14 Pro is almost 4 years old and at 65% capacity. I don't know how everyone else is (not) using their phones.

iPhone 11 coming up on 7 years old in December, still at ~85% capacity.

Really it mostly depends on how heavily you use it, and maybe to a lesser extent charging habits (charging slowly, not charging to 100%, etc).

I have an iphone 13mini at 5 years and 81% capacity.

With reason. That glue is a nightmare to deal with. I would not advise anyone start with iPhone repair.

I damaged my first iPhone repair even after multiple years of active hardware modification on other consumer devices. The older models really only go smooth if you have the Apple tools. A nice $149 of tool rental to swap the $50 part at home, rather than paying the $199 to get Apple to swap it for you. Odd how that math works out.

I took an iPhone SE generation three to Apple Store to replace battery and they broke the NFC functionality in a way that passed their simple diagnostic tool but failed to work in the real world. I had to go back and reset the phone and upgrade my IOS so they could troubleshoot it (the phone menu for setting appointments does not allow you to pick repair broke NFC funny enough) and they were nice enough to give me a new iPhone SE to replace we proved it was broken. But that was kind of worrisome and put me off any kind of Apple self repair for something they still make.

My 15 pro bought on release is at 91% and 607 cycles, for more reference.