Lydia Birk, 56, has held on to her favorite copy of “The Velveteen Rabbit” since her three children — now in their 20s and 30s — were young.

She loved being a stay-at-home mother, and filled her family’s home with books. (All of her children could read before they started school, Ms. Birk recalled with pride.) She hoped one day to be a “cool” grandma who would share her favorite stories with a new generation.

But none of her children want to have kids. And though that decision is “right for them,” Ms. Birk said, it still breaks her heart. “I don’t have young children anymore, and now I’m not going to have grandchildren,” she said. “So that part of my life is just over.”

Like Ms. Birk, a growing number of Gen Xers and baby boomers are facing the sometimes painful fact that they are never going to become grandparents. A little more than half of adults 50 and older had at least one grandchild in 2021, down from nearly 60 percent in 2014. Amid falling birthrates, more U.S. adults say they’re unlikely to ever have children for a variety of reasons, chief among them: They just don’t want to.

“That is a best and worst thing about having kids,” said Ms. Birk’s husband, John Birk Jr., 55. “You watch them make their own decisions, different from your own.”

Still, would-be grandparents like the Birks may experience a deep sense of longing and loss when their children opt out of parenthood, even if they understand at an intellectual level that their children do not “owe” them a family legacy, said Claire Bidwell Smith, a therapist based in Los Angeles and the author of “Conscious Grieving.” It doesn’t help that our society tends to paint grandchildren as a reward for aging.