Why would many people have degenerative disc disease and others do not?

My ex is 74; his girlfriend of twenty years is 59. Both of them have degenerative disc disease and I don’t. Why is that? He has pain from his hip to his ankle and she wants back surgery . Neither one of them work outdoors. He is a Ph.d and she is getting her Ph.d after she recovers from two weeks in the hospital with 30% lung function and some infection that went to her heart. She told me she can barely bend down to put clothes in the dryer. I can touch my toes with no problem. Do they just need a new mattress or can this be caused from major stress which they’ve both had for twenty years? Is this possible?