Whether the Eagles planned it or not, the strength of the defense is also their youngest unit. In fact, their defensive line is the youngest in the NFL with an average age of 24. And a byproduct of that youth has been a healthy competition of sorts.
"We try to push each other," Thornton said last week. "Sometimes it works. Sometimes it takes a little while. But we don't talk about that much. We just come out and work every day."
The play of the line has spoken volumes about the work put forth by defensive line coach Jerry Azzinaro and his charges. The season has been a roller coaster for the team overall, but the defense has steadily improved, with the line the most consistent performer.
There are five games to play and the Eagles' playoff fate is undetermined, but no matter the end result, coach Chip Kelly and general manager Howie Roseman have to feel pretty good about their defensive line beyond this season.
Thornton, at 25, is the elder statesman among the starters. But he's only in his second full season. Cox, remarkably, is just 22. And rookie nose tackle Bennie Logan, recently promoted after veteran Isaac Sopoaga was traded, is 23.
The second team is slightly older, but everyone is 25 and under. Clifton Geathers and Vinny Curry are 25. Rookie Damion Square is 24. And Joe Kruger, who is spending his rookie season on injured reserve, is 21.
There isn't a likely Pro Bowler in the group, although the argument could be made for either Cox or Thornton. But given the week-to-week improvement, one has to wonder: Have they even scratched the surface of their potential?
"I don't know the answer to that," defensive coordinator Bill Davis said. "The ceiling always has a lot of variables with it. I love where we're starting. I love the direction and how fast we're climbing with the young group.
"The ceiling, I don't know. I've seen a lot of guys take big steps in years two, three, four. I've seen guys hit a slump in two, three, four."
Cox had a tough time early in the season. The Eagles' top draft pick in 2012 was struggling, particularly against the run, while Thornton was having greater success making the transition from a 4-3 to a 3-4.
Asked to name his best lineman on several occasions, Kelly always mentioned Thornton. He said Thornton was his best pass rusher even when Cox led the team in hurries and sacks. It was almost as if the Eagles were trying to motivate him.
Thornton's hustle also seemed to spur Cox.
"Young ones like us are always competitive," Cox said. "That's what keeps that energy going in that room."
The group is made up of high draft picks like Cox and Curry, mid-round selections like Logan and Kruger, undrafted guys like Thornton and Square, and a journeyman in Geathers.
"They're not learning any bad habits from old D-linemen," linebacker Connor Barwin said. "They all come into this year open-minded, eager to learn. They're all young. They're all still fresh and play hard. And they all want success."
Sopoaga, 31, was signed to a three-year contract in the offseason, but he was jettisoned to the Patriots in late October. That gave Logan the opportunity to play more.
He has thrived with more time. Square is no longer inactive on game days. Geathers has shown flashes since Sopoaga departed. Curry, still not ideally suited to two-gap football, isn't as much a liability against the run.
But Cox and Thornton are the linchpins. A week ago, Thornton was second to only Houston's J.J. Watt in run stops among 3-4 defensive ends, according to Pro Football Focus. The numbers don't reflect it - the Eagles are 21st in the league in rushing yards allowed - but the team has been stout against the run.
Cox has 19 hurries and three sacks, but aside from Curry, he has gotten only marginal pass-rushing help from the Eagles' other linemen and linebackers.
The Eagles may not have "dominant" linemen like Jets end Muhammad Wilkerson or Chiefs nose tackle Dontari Poe, but Cox has the ability to develop into one, as Davis suggested.
Azzinaro, a former Oregon assistant, has seemingly taken the correct approach with such a young group.
"All he talks about is the sled," Thornton said, "and defeating the person in front of you."
And if there's a little extra drive to outwork the teammate next to you, that's fine, too.