I've heard that if you look at _successful_ startups, not necessarily unicorns but ones that grow into solid companies on a firm financial footing, the average age of the founders was in the middle 40s.
As we were clearly made aware of in the last year, many unicorns of the last 5-10 years were little more than good marketing and a Tier 1 VC footing the bill.
And if you’re a VC, weighing by market share makes sense.