If that is the case, and the Marek's kills around 2 years old, then only breeding from birds around 2-3 years old would improve resistance because any birds that don't make it that long weren't resistant and birds that make it that long are resistant.
Yep, only my chickens threw a monkey wrench on that option. I had a Welsummer rooster die at 25 months. True to the course of this disease in my flock, he was fine one day, the next I found him sitting on his roost, head down, gasping for breath and cyanotic. I put him down so he wouldn't suffer. No other symptoms other than being thin. I suspect either lung or heart tumors. He had developed a limp when he was very young. Looking back, it was probably the initial MD leg involvement that he survived only to succumb to the tumors when breeding season was rolling around.
I'm shooting for any birds who will survive longer than 25 months at this point. I do have hens that meet that criteria but unfortunately none of my roosters have made it that far. If by next spring I have any surviving roosters, I'll start hatching some eggs from my standards. Right now their breeding program is on hold until I can get a rooster or two with some resistance and longivity.
I find it ironic that I love roosters dearly and they are the ones that succumb fastest for me.More than likely due to breeding stress.
My boys are very special to me and at this point none of my original 4 roosters have survived. Luckily I have a high hatch rate of cockerels in my bantams. One will be a year old in September. Half way there!