"Never work with animals or children." W.C. Fields.
I'm going to swim against the stream here and say I think the adult versions of the Losers are just as fleshed out as their younger selves. If anything, I think they are more clearly defined. The problem is that what we want and what we can relate to aren't always the same thing. We long for the simplicity of youth, before time grinds us slowly inexorably into dust. A lucky few remain young at heart, and are largely untouched (or more accurately unscathed) by the frost of time. The story is about recapturing youth just as much as it is about what happened to them as youths. It is only natural that when held up next to each other we would long for (and thus like) the younger losers better. Of course kittens are cuter than cats. Of course puppies are more adorable than dogs. Do you get my point?
Young Bill has the clarity of youth. He is basically a good person on top of that. Adult Bill is a storyteller, a writer. As most writers will tell you, the most exiting things they do are almost always in their own minds. Romantic characters like Hemingway, whose lives rivaled about which they wrote, are the exception, not the rule. That makes it an unfair comparison. Young Bill is thrust into an epic war at the peak of his purity. Not unlike young guys (pretty much children themselves) thrust onto the beaches of Normandy during World War II, young Bill can't do anything but inspire. For him the options are simply "stand and be true" or not. Adult Bill has to deal with all the crap you and I deal with. The options to be a hero are few and far between for people not facing down immortal, evil monsters. We pay the bills, try to be good people, and keep passing the open windows. Do you follow? So how can adult Bill not seem to suffer by comparison to those of us reading?
However, I submit that adult Bill has something that is to be admired (all of the adult Losers do). He knows better. Bill is no longer a boy playing with a toy sword. He isn't some young kid storming the beach at Normandy who doesn't have any idea what war truly is about. Adult Bill has been to war. When he goes back, and more and more memories become available, he has a harder choice to make. It is easier to "stand and be true" when you don't fully understand the consequences. It is easier to do when you still feel immortal as all children do. Consider how much harder a choice it is to make for adult Bill, for all the adult Losers, than it was before. It goes without saying that I've read the book many times. When I first read it, my natural inclination was much like others here; I favored the parts about the younger Losers. That changed with each rereading of the story. I now relish every word and the adults and their chapters mean just as much to me. To dabble further in metaphors, the interlocking stories are of two flavors, sweet and savory. You have to have them both to be fully satisfied.