FLOWERY BRANCH, Ga. — The Atlanta Falcons traded experience for youth when they rebuilt their defense.And the young squad will be tested early.The Falcons play the Miami Dolphins in Sunday's season opener. The Dolphins, led by running backs Ronnie Brown and Ricky Williams, ranked 11th in the NFL in rushing last season. The Falcons, on the other hand, were 25th against the run."We are a very young defense," said coach Mike Smith. "We got significantly younger this year and we are going to be playing a lot of players that have three years or less in terms of experience."The defense lost many of its most experienced players from last year, including five starters: linebackers Keith Brooking and Michael Boley, safety Lawyer Milloy, defensive tackle Grady Jackson and cornerback Domonique Foxworth.Mike Peterson, entering his 11th season, was signed to replace Brooking as the leader at linebacker, but the other new starters are significantly younger. Defensive end John Abraham, 31 as he begins his 10th season, is the only other projected starter who is 30 or older.The list of new starters also includes defensive tackle Peria Jerry, a first-round draft pick, and third-year linebacker Stephen Nicholas, who has never started.Thomas DeCoud, a second-year player, will make his first start at free safety. Third-year players Chris Houston and Brent Grimes are the projected starting cornerbacks.Curtis Lofton, who had 108 tackles as a rookie last season, is a crucial part of the run defense at middle linebacker.Smith acknowledged he is concerned about having so many young starters."I'm really concerned about how our young players are gonna respond to their first significant action here in the league," Smith said.The changes have left safety Erik Coleman, who has 66 starts in his first five seasons, as one of the new leaders."You have a lot of young guys now and with me being one of the older guys in the secondary, a lot of young guys come to me with questions," said Coleman, who was second on the team with 127 tackles last year. He also had three interceptions."I'm fortunate to have played a lot of football and I just try to do my best to help them with what I've been through and what I've seen," Coleman said.Peterson said the defense can't be judged on one game."I think the question is going to be answered over the course of the season," said Peterson when asked about the run defense against Miami. "You can't really judge a team off one week. You're definitely going to get a sneak peak of what it's going to be, but you judge a team and you judge a player off a whole season."We're definitely going to get tested and we're definitely going to see how we hold up."The defense struggled at times in the Falcons' 2-2 preseason. San Diego, playing without LaDainian Tomlinson and Antonio Gates, converted 7 of 8 third downs in the first half of a 27-24 exhibition win at Atlanta on Aug. 29.Abraham said the young players can learn from mistakes in exhibition games."Sometimes you got to get a little negative before you get positive, and I think that's really helping us," Abraham said.More changes could be coming in the secondary. The Falcons made two moves in the last 10 days to add experience at cornerback.Atlanta acquired Tye Hill from the Rams on Sept. 1 and signed eighth-year veteran Brian Williams on Sunday. Williams played for Smith when Smith was Jacksonville's defensive coordinator. Williams started all 16 games with the Jaguars last year, splitting time at strong safety and cornerback.Smith said he has tried to give Hill and Williams as many snaps as possible this week, but it's unknown if either will play against Miami.Rookie William Moore, a second-round pick, was expected to compete for Milloy's spot, but he has been slowed by injuries. He had arthroscopic knee surgery in training camp and then suffered a strained hamstring in the third preseason game against San Diego and has not practiced this week.Chris Owens, a third-round pick, and Chevis Jackson, a second-year player, are the other options at cornerback.