I think a lot of this comes from the superstar model implicit in the VC ecosystem. One perennial theme in the SV approach to business is the idea of making a lot of bets with skewed upside. Like buying call options, the vast majority won't pay out. But if you do it right, a small handful of homeruns will make up for the rest.
I think this mindset easily carries over from the investing side into the hiring side. What is an employee, if not a risky upfront investment? Especially at a hyper-growth startup that's already bleeding cash flow. Very few 23-year olds will be productive enough to justify a quarter million in compensation. But a handful of very talented ones, when put in a high-impact position with a ton of responsibility will knock it out of the park.
The median 40-year old experienced engineer is almost certainly more competent and productive than someone fresh out of school. But Silicon Valley doesn't optimize for the median, it optimizes for fat right tails. Hire or invest in a 23-year old SWE and there's always a small chance that you be scooping up the next Elon Musk. At 40, Elon Musk definitely wasn't browsing through monster.com.
However, I believe this paradigm rests on a faulty premise. A more reliable way to create an exceptional organizations is to build an exceptional culture, not try to bottle the lightning of individual superstars. Unfortunately, that approach involves sacrificing some sacred cows of Valley ideology.
Building a strong corporate culture requires effective managers, as well as brilliant engineers. The typical founder does not fit the mold of an effective corporate manager. 28-year olds can be good managers, but a 50-year old with decades of experience is almost certainly more likely. Replacing the founder with a professional CEO is anathema is the modern "founder-friendly" culture.
Beyond that a professional manager at the helm very likely means a more button-down corporate orthodox environment. Not necessarily because it's better, but just as a byproduct of how the CEO is used to operating. Mitt Romney runs a very different shop than Adam Neumann. Put Mitt in charge of the next startup, and things are going to start looking uncomfortably more like Initrode rather than the typical hipster startup.