I don't think it's particularly a "conspiracy theory", in that there's no cabal of people trying to cover this up. It's more an optimization function; we have N productivity units, and M humans (where M > N), so we hire from the most productive subsets of M. For the less productive subsets (largely youths, criminals, geriatrics), we encourage institutionalization to minimize the portion of M-productive required to maintain the unproductive ones.
If you look at post-industrialization patterns, the age at which humans become productive has been increasing as the productivity of humans has increased. On a subsistence farm, a 6-year old can be doing useful work to increase the productivity of the farm. In a coal mine or factory, an 8-year-old can do productive work. 16-year olds can do construction, food industry, but for jobs with decision-making responsibilities (as more and more jobs are today), we don't trust anyone under 22.
If you look at post-industrialization patterns, the age at which humans become productive has been increasing as the productivity of humans has increased. On a subsistence farm, a 6-year old can be doing useful work to increase the productivity of the farm. In a coal mine or factory, an 8-year-old can do productive work. 16-year olds can do construction, food industry, but for jobs with decision-making responsibilities (as more and more jobs are today), we don't trust anyone under 22.