I have a 12 Pro and not that old. Last night it appears the battery didn't charge, it showed on the little icon when I plugged it in that it was charging, but this morning only 22% and I started with 51%.
So I did what I have done in the past when I had my 8 plus when it happened. I shut off...
Halfway through my trip in Maui, Hawaii (no Apple stores on the island) my iPhone 6 decided it was going to stop charging. It simply would not do anything when I plugged it into my lighting cable (which is a Apple product). I even tried it with my wife's cord and it still wouldn't charge...