I would take it to mean that artists that are younger are far less constrained by rules. In your teens and twenties, life is about bending or breaking rules. This can lead to amazing art.
In your 30's and 40's you are learning the rules, the meaning and reason of them, you are learning about art. There is a consciousness that it is near-impossible to develop at too early an age, where reality of life and experience intrudes on all aspects of thought and whimsy.
At 50+ (and obviously, this is all vastly over-generalising, like the quote), there are the art and life lessons learned, humility (which Jobs never really mastered), a sense of place and purpose and a knowledge of self that allows, or should allow, art to 'happen'.
I would liken it to seeing one of those crazy funky caterpillars turn into an amazingly brilliant and colourful butterflies. The transformation is absolutely necessary, but it doesn't happen without the dull, somewhat ugly cocoon wherein one matures.