A question was posted a couple of pages ago, asking who defined an "average pilot", taking issue with the "tone" of the Boeing statement about not giving too much info to said "average pilot" and implying the airframer was somehow in favour of the idea of a "lowest common denominator" pilot.
Nothing, from my perspective, could be further from the truth than that last item. But to address the points raised:
The concept of the "average pilot", or terms very similar to that, is found in many places in part 25. Picking a regulation at not-quite-random, 14CFR25.143 (b) states (my emphasis)
(b) It must be possible to make a smooth transition from one flight condition to any other flight condition without exceptional piloting skill, alertness, or strength, and without danger of exceeding the airplane limit-load factor under any probable operating conditions, including
You'll find similar wording throughout the regulations where the ability of the pilot to do a task will influence whether you pass or fail the requirement. The purpose of these words is to prevent the Chuck Yeager of any specific company going out, flying some ridiculously perfect and intricate manoeuvre, and saying "it's fine, I can fly it, must be ok". Test pilots are, for the purpose of assessing the piloting demands of the aircraft, not allowed to use their innate flying skills (they can use them for the "academic data gathering manoeuvres, where pilot workload and human factors are not a criteria) - instead they have to use their TEST PILOT skills - and specifically the one which says "I can determine, from my knowledge of how pilots in general interact with aeroplanes, and what they can and cannot be expected to manage, that this is/is not acceptable". The term frequently used for this is "average pilot".
Who decides exactly what the average pilot can do - collectively, the Test Pilot community; it is ultimately a judgement, but one made by a group, including not just the company's own pilots but also the Test Pilots from the various regulatory agencies. And they will indeed base that judgement on an understanding of the target pilot for the machine in question - so a private pilot won't be assumed able to cope with as much as an airline one. And as awareness builds of shifts in the pilot population and their skillset - as in, for example, many fewer with a military/fighter background, the skill level assumed may be degraded or amended, OR it may be identified that remedial action in training etc is required to bridge the gap. Usually it's all of these things.
And why do I say the OEM has no interest in downgrading the average pilot? Look at the first point - the smarter the "average pilot", the easier it is to pass the cert tests, even with a really clunky and horrible human interface or terrible flying characteristics. We'd love to assume every pilot was the hero who saves the day - every failure would be classed as benign, and certification would be a breeze. But that's not realistic, nor frankly fair to the people we hand the machine over to when the cert is done.