But their most grievous indiscretion is showing up at this dance with young squads -- after showing up far more veteran teams on the way. In fact, were this a real dance at a night spot, the Rockies' and D-backs' starting lineups would have to be carded.
Even in the 1960s, when one wasn't supposed to trust anyone over 30, people would have had infinite faith in these teams.
Colorado's starting nine in Game 1 will include only two thirtysomethings: Todd Helton (33) and Kaz Matsui (31).
Arizona's lineup will be even younger, with left fielder Eric Byrnes (31) the only player out of his 20s.
So welcome to the new fall miniseries, The Young and the Reckless. Welcome to baseball's new wave, which in two weeks will wash one of these teams ashore in the World Series. Welcome to fresh, novel, exciting.
"We're all kind of sick of hearing that we're too young to win," said Arizona first baseman Conor Jackson. "Especially the last two months of the season, we heard a lot of that -- when we had a bunch of guys get hurt, like Orlando Hudson and Chad Tracy.
"We weren't picked to win anything. That's fine. That's motivation for us. We'll take it and roll with it."
Jackson plays on one of only two teams where being 25 in your third big-league season makes you a clubhouse elder.
Colorado, of course, is the other.
"We have a lot of young players to lean on, just like the D-backs," said Jeff Baker, the Rockies' versatile 26-year-old utility man. "Everyone in this clubhouse is really, really loose. No one is above anyone else. Everyone gets needled the same."
The perception is that the two opponents make this NLCS the youngest postseason series in history. There are many ways of considering that picture, some subjective.
Adding to the demographics of the starting lineups, for instance, is the fact that the four-man rotations for both teams average 27 years of age. The closers are 27-year-old Jose Valverde and 24-year-old Manuel Corpas.
Also: The D-backs have 10 players 25 and younger, while the Rockies have eight; five players on Arizona's postseason roster started this season in the Minors, including one in Class A (Justin Upton) and another in Double-A (Mark Reynolds); and a half-dozen players on Colorado's postseason roster came up from Triple-A Colorado Springs.
But here is an indisputable inset:
The ages of the two starting shortstops, the men fielding the most responsibility at the most vital position on the field, add up to 47, believed to be the lowest total for a postseason series. Colorado's Troy Tulowitzki, who turned 23 on Wednesday, and Arizona's Stephen Drew (24) could thus be the cover boys for this theme.
Subtitle the NLCS: Who are these guys, and what are they doing here?
Both clubs could have had a field day with the media during Wednesday's workouts at Chase Field, had they wished to be mischievous. The national media invading the locker rooms were, for the most part, having their first get-together with these players, whom they could identify only by name plates above lockers.
The players could have swapped lockers, and few reporters would have been the wiser.
"These guys should be wearing name tags," someone facetiously suggested to Jay Alves, the Rockies' vice president of communications, who nodded.
"Yeah, no one knows them," Alves said, then quickly added with a wink, "But they will, soon enough."
The two teams are reflections of each other, to an astonishing level. No wonder Jackson called the Rockies "our brother squad" and Brad Hawpe called the D-backs "our neighbors."
They fought the same uphill battle to finish with the same number of wins, and competed at the most critical time with youthful rosters that flourished thanks to the same two factors: familiarity with each other, and support from the few veterans around them.
"The core of our team has been together for years now, starting at the Minor League level," Baker said. "We're homegrown, with homegrown veterans who help with the leadership sprinkled in."
That's precisely the same thing one hears in the other clubhouse, where the majority of the players came through the Triple-A Tucson pipeline.
"We've played together for a while now," Jackson said. "I played with Drew and a bunch of other guys. We all know each other's swings. When I'm in a slump, I can go to a guy who has seen my swing from Day 1. We're lucky to be in this spot -- it's a treat for all -- but we created bonds when we were in the Minor Leagues, coming from scraps, and that's when you really get to know a guy."
"We have a lot of young players to lean on, just like the D-backs. Everyone in this clubhouse is really, really loose. No one is above anyone else. Everyone gets needled the same."
-- Rockies utility man Jeff Baker
Upton recalls his reception when, after quickly advancing through Class A and Double-A, he joined the D-backs in midseason as a 19-year-old. He met the club in San Diego, where manager Bob Melvin called him into his office for a brief one-on-one.
"There will always be eight other guys on the field with you," Melvin told the young outfielder. "Don't feel extra pressure. Just play your game, make your adjustments, and you'll be OK."
"Bob's a quality guy," Upton said. "He's great at giving young guys support."
The Rockies have a more integral steadying force in Helton. His mentoring counterpart on the D-backs is Tony Clark, the 35-year-old first baseman who is not a regular.
"I've been amazed by how calm and cool this club is," said Seth Smith, the 25-year-old outfielder who joined the Rockies out of Triple-A only three weeks ago. "Everyone feels comfortable, no one is rattled by the atmosphere or how big the games have become.
"A lot of that has to do with Todd Helton and Matt Holliday, how they go about their business. All the younger guys feed off them. The stars on this team are even-keel guys, and we feed off that."
Inertia has brought both youth-dominated clubs to this point. They survived taut races into the postseason, the Rockies having to go the extra lap of a playoff game with San Diego, and breezed through Division Series sweeps.
But it isn't as easy, and is a far greater accomplishment, than they have managed to make it look.
"I think our core is a lot younger," argues Arizona's Jackson. "We've got a lot more guys with that deer-in-headlights look. Three guys from Double-A? That's not an easy thing to do, going from Double-A to the big leagues. It's like jumping from the sixth grade to a high-school senior, going from arithmetic to calculus."
It doesn't compute, But the Young and the Reckless compete.