In almost every case, the best practitioners of any profession have experience in their field.
You can't learn things from books alone. There is a lot of implicit knowledge in any field that is either not well documented, or perhaps can't be well documented. Many things you can only learn from direct experience, or by learning from observing others who have been around the block.
The amount of experience necessary to be a first-class practitioner can vary from field to field and person to person.
But one thing I've observed is that a lot of experience doesn't necessarily equate to a high degree of competence. There are times where the more experienced a person is, the more dangerous they are. (Especially if that lack of competence is compensated by the confidence developed through working in the same field for a number of years.)
Someone with 20 years' experience is an environment that doesn't encourage professional development, or doesn't encourage people to challenge their assumptions or reflect on what they are learning on a regular basis, will not have accrued the same benefits as someone who has developed their skills in expertise in an environment that encourages these things.
Likewise, someone with a mindset focused on professional development and contemplation is far more likely to develop significant competence over a long period of time than someone who pays little attention to such things.
Twenty years is also a long time to reinforce bad habits and behaviours, and get stuck in certain unproductive ways of seeing the world.
Experience can be a great teacher. One of the best teachers there is. But there's no guarantee that this is the case.
One of the best illustrations of this is Hurricane Katrina. Amanda Ripley discusses Hurricane Katrina, which hit New Orleans in 2005, in her fantastic book, The Unthinkable. Ripley refers to analyses suggesting that “the victims of Katrina were not disproportionately poor; they were disproportionately old. Three-quarters of the dead were over sixty…. Half were over seventy-five."
There's an interesting theory about why this might have been the case:
"They had been middle aged when Hurricane Camille[, a similarly large Hurricane that approached New Orleans in 1969, but narrowly missed the city and caused less damage than was expected]. ‘I think Camille killed more people during Katrina than it did in 1969,’ says Max Mayfield, director of the National Hurricane Centre. ‘Experience is not always a good teacher.’”