It's significant but not necessarily the most productive.
What makes it significant is our evolution of developing our own life philosophy through various ways of learning and through trial & error from mistakes. Eventually we come to realize (who we are) and what it is we want or need to truly feel happy.
Youth is also a time where we believe in the possibility of our dreams at the highest level. We haven't had to deal with as much heartache, rejection, and may in fact be naïve in the ways the world operates. We're more trusting and less cynical. When we said "nothing is impossible" we believed it!
The person who discovers their true calling early in life is likely to have a happier and more prosperous life!
Most people however look back on their youth with "regret" because they spent large chunks of time and money on meaningless endeavors like partying, getting laid, and flying by the seat of their pants. We acted as if we had a 1000 years to get it together.
The so called "midlife crisis" or "midlife awakening" comes when we truly realize our time on earth is not forever. If we are to live to see 80 it's easy to do the math. The 1st quarter ended at age 20, the second at 40, the third at 60 and beyond that we enter into the fourth quarter. It's also within the third quarter we start to see that some of our fellow classmates and friends have already died.
We now break out our "Bucket Lists"!
Many of us wish we would have put more time and energy into our careers and amassing wealth so that the second half of our life would be more secure and provide us with more (options).
Instead many of us find ourselves living on fixed incomes which limit our happiness or we're working on jobs we dislike or worse in search of a job. It's too bad wisdom comes after youth passes.
Our "golden years" are not in fact "golden" at all.
Lots of people end up staring out the window reflecting on their lives wondering where all the time went and how they would have done things differently. George Bernard Shaw probably said it best.
"Youth is wasted on the young."