It is sometimes said age is just a number, and that might be true for the NFL's ageless wonders such as Tom Brady, 40, or Drew Brees, 38.
But for most mortals who play in the NFL, age is significant. Get to 30, and diminishing skills and inflated paychecks will push players out of the league.
Age matters for teams, too.
A young roster is cheap, filled with players on minimum-salary rookie contracts and little ability to dart elsewhere in free agency. The future might be bright, but some of the NFL's youngest players get sidetracked. On- and off-field mistakes are bound to happen.
Older rosters have savvy, battle-tested players. They generally know where to be, what to expect and how to act. That experience comes with a price that weighs down even the NFL's ever-expanding salary cap. Replacing a serviceable 10-year veteran with a rookie can create cost savings of 50 percent or more.
The Arizona Cardinals have the oldest group of 22 starters in the NFL, based on actual lineups used by teams in Week 1 games and projected lineups for the Dolphins and Buccaneers, whose opener was postponed because of Hurricane Irma. The Cardinals' average age of their starters was 28.41 years old, or 0.77 years older than any other team.
Arizona has eight starters who were at least 30 years or older on Sept. 10: quarterback Carson Palmer (37), linebacker Karlos Dansby (35), wide receiver Larry Fitzgerald (34), defensive lineman Frostee Rucker (33), center A.Q. Shipley (31), tackle Jared Veldheer (30), guard Mike Iupati (30) and safety Tyvon Branch (30).
On the opposite end of the spectrum, the Cleveland Browns have the NFL's youngest group of starters. Their average age of their starting 22 players on opening day was 24.4 years old, or 1.6 years younger on average than any other team in the league.
The Browns had only one starter, tackle Joe Thomas (32), who was at least 30 years old in Week 1. Cleveland had three 21-year-olds starting (quarterback DeShone Kizer, tight end David Njoku and safety Jabrill Peppers), as well as four 23-year-olds and five 24-year-olds. That means 12 of their 22 starters -- more than half -- were 24 years or younger on opening day.
Offensively, the Saints have the NFL's oldest group of starters, due in part to Brees' advanced age. They also started four other players who were at least 30 in Week 1: tackle Zach Strief (33), running back Adrian Peterson (32), wide receiver Ted Ginn Jr. (32) and center Max Unger (31).
The Browns had the NFL's youngest offense, followed by the Seahawks. Tight end Jimmy Graham was the Seahawks' only offensive starter at least 30 years old.
On defense, the Seahawks were among the oldest teams in the league, although their oldest starter was defensive end Michael Bennett, who is 31. The Dolphins' projected starting lineup was tied as the NFL's oldest, with three players 30 or older -- Cameron Wake (35), Lawrence Timmons (31) and Ndamukong Suh (30) -- and only two players under 25.
Defensively, the Browns had the NFL's youngest group of starters. The Saints had the second-youngest, although New Orleans got a boost by having only 10 players on the field to start Monday night's game against Minnesota, according to the NFL's official game book and a review of the play on television. The Saints had a pair of 21-year-olds starting on defense, and their oldest defensive starter was 28-year-old Cameron Jordan.
An overall look at the average age of each team's starting lineup in Week 1: