Not that I'm arguing that a young team can't or won't win the cup, however, how many "young" teams have won the cup...like, ever?? Even the teams making it deep in the playoffs tend to have a healthy mixture of experienced vets sprinkled with youth.
I think people are confused by the fact that because we're quite youthful, we should have energy for days and be able to outlast older experienced teams. That's not always the case and you can't discount the value of experienced and savvy veteran players.
Having said that, looking at the age of the Leafs, they are right there at that point where things should start to click as our core matures a little bit more. The supporting cast, for the most part, are in their primes so if the team remains constructed as is, we should start to see positive results no later than next season, I'd wager.