Who's next to retire?

As I was watching Tony Stewart's Press Conference I couldn't help but think what other drivers will be hanging up their helmets within the next few years. Greg Biffle comes to mind. He's 45 years old & has been with Roush since the start of his NASCAR career & hasn't had much success in NASCAR these last few years. Others are Jimmie Johnson, Dale Earnhardt Jr & Matt Kenseth. All are over 40 years old & may or may not think about hanging up their helmets within the next 4-5 years. All have had tremendous careers just like Tony Stewart & I'm sure they'll keep themselves pretty busy when they do retire.