In theory, the strings are easiest to bend closest to the center of their length, which is always right around the 12th fret. Any difficulty you have at ~the 16th fret should be about the same as about the 9th fret or so. If it’s more difficult for you higher up, then it could have something to do with your “angle of approach” and would just be a matter of developing the right muscles by repetition.
Although if you’re playing an acoustic, there’s an entirely different set of problems playing that high up the neck
.