You are right. I was thinking at a young age though. How many people are working with professionals in a given field while still in elementary school? In fact, in many places there are laws against working with those people at such an age, often with farming being the only exception. Adults do have more options, including college, which is possibly the de-facto standard to make up for what I suggest is lost as a child.
> imagine someone self-taught building their own reaping machine for your farm, would you try it or would you trust better someone with a more engineering background?
If you look at the history of the farm equipment manufacturers, they all pretty much started on farms to solve the problems of the farms, built by the farmers. Today, in the well established market, you do not really know who is building the products. They might be engineers, they might not be. Nobody asks - nobody cares.
Aside from nobody using reaping machines anymore, if someone self-taught brought a new reaping machine onto the market, I'd certainly evaluate it. It is not automatically any worse than anything else available. Some of the agriculture products built by big-name manufactures (and presumably fully trained engineers) are laughably bad to begin with, so the bar isn't necessarily high anyway.
Today, it doesn't make economic sense for a farmer to build a machine from scratch, but farm-built modifications to production equipment is very common. While some farmers are professional engineers, farmers come from all walks of life. It doesn't really seem strange at all to be building that kind of thing without formal training. It is just part of the job.