The average age of a player for the Diamondbacks this year was 28.5 years old. The average age for a Cardinal this year was 29.6 years old. While one year doesn't seem to be much different, the teams were 5 positions away from each other in all of baseball just last year.
Youth won't necessarily lead a team to the playoffs, and veterans won't either; a combination of both is ideal for a team. The best teams have wily veterans, stars in their primes, and young players with athleticism and drive. The Diamondbacks have a good combination of these items, particularly young players.
Of the 9 players on the Diamondbacks who have the most playing time this year, 4 of them have 1 year or less of service time in the majors. Their 4 bench players according to Roster Resource all have 1 year or less of service time. The third-best starting pitcher in their playoff rotation is a rookie. Three of their key bullpen pieces are all rookies. Suffice it to say, the Diamondbacks are a young team with various rookies making key contributions in the playoffs.
The Cardinals have 4 players with 1 year or less of service time making contributions at the major league level as well. However, their top three starting pitchers all have at least 4 years of service time in the majors, and virtually none of their bullpen pieces have less than 1 year of service time accrued. The Cardinals would be wise to follow in the footsteps of the Diamondbacks and lean into the youth infusion that is happening across baseball.