A forty year old has an inclination for success and is therefore suited best for companies in the growth phase.
Twenty year olds seem to be naive enough to go for the impossible.
Or at least that's the mass perception. I think this also might be the reason for this pseudo-agism. (of course Elon Musk if the perfect counter example)
I equate 20 vs. 40 with Entrepreneur vs. MBA. Same perceptions, similar stereotypes, same utility.
That said, I love it when a person 20 years older than me actively guides my inclination towards "big" without stifling it, regarding it as idiotic, or being indifferent to it. I LOVE partnering up with older folks - they are professional in the true sense of the word - they can do what they do even when they are having a bad day.
And they like partnering withe me because, I think, in me they see themselves when they were this old. It's amazing how much you can learn from people who've been there, done that!