The same reason that the 55 year old workhorse who’s been stocking the shelves at Safeway won’t give 2 f***s about what the newly promoted 18 year old “Produce manager” has to say. It’s a tale as old as time in most aspects of life.
Competence and value play a role, and if you're a star, no one cares about your age.
Do you think Sergei Brin and Larry Page (Google founders) had trouble earning the respect of employees at Google even though they were in their 20's? Or Mark Zuckerberg at Facebook?
And before the argument is made that they were the bosses, keep in mind that the people who joined those companies were often highly paid older executives with lots of other job opportunities who often took pay cuts to follow the lead of the younger guys because driven people want to win and they'll follow the lead of people with the skills to take them to the promised land.
When I was growing my company before we had formal authority levels the team naturally took the lead of those who were the most driven, smart and skilled, regardless of age, and in both sales and software side it occasionally happened that the leader that emerged were younger. We eventually had to formalize that leadership structure because the de facto leaders often wanted to enforce things that they knew needed to be done in order to set the culture and processes right but they felt they were overstepping their bounds until we told them that in fact that was now their job. Some wanted it and some didn't, some thrived and others stumbled, but age was never the deciding factor.