Age is just a number, but it's certainly an important one in baseball.
That's why MLB.com compiled the Opening Day rosters for each of the 30 Major League teams, then looked at the ages for all 750 players. Here is a look at how the clubs stack up overall, as well as broken down into position players, starting rotations and bullpens.
It's important to note that a player's age for this season is considered to be his age as of July 1.
POSITION PLAYERS
Oldest: T-1. Mariners 30.7, T-1. Yankees 30.7, 3. Braves 30.6
Youngest: 1. D-backs 26.5, 2. Twins 27.2, T-3. Astros 27.4, T-3. Brewers 27.4
Points of interest
• While Arizona made a big splash this offseason by paying up for Zack Greinke (age 32), the club's postseason hopes are resting largely on a young core. Only one position player, 33-year-old backup Rickie Weeks Jr., is in his 30s, while every other team had at least three 30-somethings on its Opening Day roster. It was telling that when All-Star center fielder A.J. Pollock (28) fractured his elbow shortly before the opener, the D-backs opted to fill his spot -- at least for now -- with a 23-year-old (Socrates Brito) and a 24-year-old (Chris Owings).

• The Mariners have made several win-now moves in recent offseasons, signing the likes of Nelson Cruz (36) and Robinson Cano (33). The Yankees have been notoriously reliant on older players. It's more surprising to see the Braves with the third-oldest group of position players, since they are a rebuilding club coming off a 67-win season. But while Atlanta has some young pitching, it is still filling gaps with veterans such as catcher A.J. Pierzynski (39) and outfielder Jeff Francoeur (32), two of the club's nine 30-plus position players, tied with Seattle for most in the league.
• The Cubs are just behind the Astros and Brewers, with an average age of 27.9, thanks to seven position players 26 or younger. That includes Addison Russell (22), Kyle Schwarber (23), Jorge Soler (24) and Kris Bryant (24).
STARTING ROTATIONS
Oldest: 1. Cubs 31.6, 2. Royals 31.0, 3. Tigers 30.8
Youngest: 1. Braves 26.0 (four pitchers), 2. Reds 26.5 (four pitchers), 3. Phillies 26.6
Points of interest
• In contrast to their young lineup, the Cubs added 37-year-old John Lackey to their rotation this offseason, giving them four 30-plus starters (tied for most in the league), with the only exception right-hander Kyle Hendricks.
• Though just a bit older than the rebuilding Braves, Reds and Phillies, the Rays (26.8 average) were the only club with no 30-plus pitchers in their Opening Day rotation. Jake Odorizzi is 26, while Chris Archer, Drew Smyly and Matt Moore all are 27. Meanwhile, Erasmo Ramirez (26) is waiting in the bullpen until a fifth starter is needed, and top prospect Blake Snell (23) likely isn't far away.

• The Mets (29.2) and Blue Jays (30.5) both have significant generation gaps in their rotations. New York has four starters 28 or younger, including 23-year-old flamethrower Noah Syndergaard, but also Bartolo Colon, who will be the first pitcher to start a game at age 43 or older since Jamie Moyer in 2012. In Toronto, knuckleball artist R.A. Dickey (41) shares a staff with Aaron Sanchez (23) and Marcus Stroman (25).
BULLPENS
Oldest: 1. Marlins 31.6, 2. Nationals 31.4, 3. Giants 31.1
Youngest: 1. Yankees 27.0, 2. Orioles 27.6, 3. Rangers 27.8
Points of interest
• The Yankees' aging lineup contrasts with a fairly young pitching staff. Their four starters outside CC Sabathia all are 27 or younger, and New York was the lone club with an Opening Day bullpen featuring only one pitcher in his 30s (nasty left-hander Andrew Miller, who is 31).
• The Nationals sought to rebuild their bullpen this offseason and opted largely for experience, as all four newcomers who made the roster are 31 or older. That doesn't count last year's controversial Trade Deadline acquisition, Jonathan Papelbon (35).

• The Marlins are the only team not to have at least one reliever below the age of 29, having added four veterans 32 or older, in Edwin Jackson, Dustin McGowan, Chris Narveson and Craig Breslow.
TEAMS
Oldest

1. Mariners 30.4
   
2. Pirates 29.9
   T-3. Giants 29.8
   T-3. Nationals 29.8
   T-5. Royals 29.7
   T-5. Blue Jays 29.7
   Youngest
   
3. D-backs 27.0
   T-2. Rays 28.0
   T-2. Reds 28.0
   T-2. Phillies 28.0
   
4. Cardinals 28.3
   Points of interest
   • The Mariners are second only to the Royals with 14 players 30 or older, led by a couple of bullpen acquisitions, 40-year-old Joel Peralta and 38-year-old Joaquin Benoit. In contrast, the D-backs have a league-low four 30-plus players, with three of them new to the team this season.
   • As mentioned, the Cubs have an enviable group of young and talented position players, but also the league's oldest rotation. In total, their Opening Day roster was more than six months older than that of the rival Cardinals (28.9 vs. 28.3), who had only three pitchers no longer in their 20s.
   • The sharply divided National League features five of the six youngest clubs but also three of the four oldest.