As a famous young person once said "Young people are just smarter.".
It appears the message here from PG is that "Younger people have intellect but perhaps not the demonstrated tenacity and drive needed to succeed, and tenacity and drive trump intellect when it comes to entrepreneurialism.
Knowing there is stuff to learn is important for those who are beginners.
It's important to see this through a lens of having an appropriate amount of potential and experience/maturity/discipline rather than a function of age. Folks who spend more time behind keyboards than learning about the experiences of others leaves them at a disadvantage of finding problems to solve that matter to others.
Being young is an advantage in some ways, and not being young is an advantage in some ways.
The more well rounded you are, while you're younger, and are able to learn from others, the greater your chances to are succeed, earlier.
There is often (but not always) a need for young people to gather experience in problem-solving as experienced by others and discovering value.
You gotta remember, the folks in their 30+'s were 20 once too, just like anyone, living in the same world of the internet absolutely being gobsmacking in possibilities. That hasn't gone away or changed, we're on the same playing field.
Actually now that I think about it this is wrong. Everyone tends to value young people more highly than older people - that's the way our society works. What can be said is that "older people tend to devalue older people less than younger people devalue older people".
Ageism? meh - that was a snag from before you kids were born.
The second study mentioned in the article, which is longitudinal rather than cross sectional, has more optimistic results for older ages, but as it says, there are problems with tracking the same people over time, such as them just getting better at IQ tests.
Regardless, the first, cross-sectional study gives clear answers to the current relation of age to intellectual ability.
Age shouldn't be a factor in hiring, only ability, but it's not unreasonable to say that, at least right now, you can expect people in their 20s to perform better at many tasks.
It's also hard not to just agree with the "young people are just smarter" statement based on intuition. When have you ever heard of a 20 year old needing help figuring out a television remote? Something clearly changed between the time that someone learned to drive a car and when they had to be shown for the 50th time how to upload a video from their camera to facebook.