I like stories like that too.  I liked Babylon 5 a lot for a similar reason.  The humans are not the most powerful race in B5, and are even on the bad guys side for most of the story.  I really loved that about Babylon 5, and my favorites episodes of B5 were the half-dozen or so that revolved around liberating earth and bringing them back on to the side of the good guys.

Nope that is not a lapse in story cohesion.

The Ur-quan were in their ritual and too preoccupied to obliterate earth.

Who who should go after earth? The Kzer-Za? The Kohr-Ah?

The Kzer-Za were for enslavement so they would not obliterate earth anyway.

The Kohr-Ah were for destruction and they would have, but they cannot due to the current doctrine which are the Kzer-Za rules of enslavement.

So in effect until a clear winner has emerged both sides have no interest or authority to "intervene".

While the Ur-quan themselves were "busy". They did have battle thralls for that sort of thing.  I was not talking about the destruction of earth, but more to make sure none of the earlier alliance races were supporting the humans.

Are you forgetting the dozen handfuls of successful German U-boat captains during WW2 who were as young as 22 in some recorded cases? I know you said specifically "hero", but I'm sure these would counter your point. All 217 of them.

Hero was the wrong word.  By that I meant the star of the story.  Or, as the shadows described John Sheridan in Babylon 5 as "a nexus, you go one way and the whole world tends to follow".  Assuming the story in the new SC is the epic "save the galaxy" type of story you would expect, I would hope to see a realistic aged character in that role.  The whole world doesn't follow a 20-year-old when the fate of humanity is at stake.  

That was what I was trying to say.