For Vietnam veteran Bob Ranieri, 66, depression after the death of his wife and brother triggered nightmares of the war.

CHICAGO — Nearly four decades have passed since the end of the Vietnam War. Bill Simon, a 65-year-old combat veteran, thought he had had long ago escaped the nightmares and flashbacks that haunted him after his return home.

“For many years, I never had any issues,” he said. He had all the trappings of a successful life: a loving wife, three children and a house in Arlington Heights, Ill. But about 10 years ago, the nightmares returned. Night after night, they became more vivid and more bizarre.

“Regardless of whatever I start dreaming about, the dream always mutates into some Vietnam incident,” said Simon, a research specialist at a petrochemical company. “They’ve gotten progressively worse. Right now, I barely sleep.”

Simon doesn’t know what triggered the return of his nightmares. But experts say his experience is not uncommon. As Vietnam veterans age, many discover they have more time to contemplate their lives. The time for reflection — as well as retirement, reunions with war buddies and the deaths of loved ones — can stir memories from a long-ago war.

An estimated 2.7 million men and women served in Vietnam. Their average age is 64, according to Vietnam Veterans of America. “Most are approaching retirement,” said Tom Berger, director of the health council at Vietnam Veterans of America. “Once they retire, their spouse has passed and the kids have left home, without that structure, they begin to think about things.”

Anniversary dates and holidays such as Veterans Day may begin to bother people. But even when a veteran seeks treatment late in life, experts say, in many cases the post-traumatic stress disorder had been there all along.

For Tim Markowski, 65, a wounded combat veteran, retirement meant more time to think about the young North Vietnamese soldier he killed while on patrol in 1967.

“He was probably about my age. He was probably as scared as I was,” he said. “After all these years, that’s when (the thought) creeps in. Who was this guy? He had a mother and a father. Maybe he was a dad. Over the years, that has come back to me.”

It wasn’t until after his retirement that Markowski went to Edward Hines Jr. VA Hospital and was diagnosed with PTSD. “We got on with our lives — I did — but it never goes away,” he said.

Memories form a complex web of images and emotions. It’s hard to know how one event might trigger recollections from decades before, experts say.

At Captain James A. Lovell Federal Health Care Center in North Chicago, more Vietnam veterans are reporting symptoms of late-onset PTSD. “I think that’s due to the fact that Vietnam veterans are at an age when they’re experiencing more loss and all the life changes that can be triggers,” said Anthony Peterson, who runs the center’s treatment programs for post-traumatic stress.

The passing of a spouse can stoke feelings of survivor guilt. A serious illness can force a veteran to confront death in the same way he once did in Vietnam.

Experts emphasize it’s never too late to get help. “We have good treatments for PTSD. If you come into the hospital and see us, we can help you,” said Dr. Matthew Friedman, director of the Department of Veterans Affairs National Center for PTSD. Studies show that treatment is effective regardless of whether a person seeks help immediately after the traumatic event or decades later.

For Bob Ranieri, 66, of Chicago, the death of his wife and his brother in the span of five months brought a long period of darkness. Depression triggered nightmares from the war. At the urging of a few friends who served in Vietnam, he went to see a psychologist and she diagnosed him with PTSD.

Now his most powerful therapy, he said, is spending time with his five grandchildren, whom he calls “my joy.”

His voice sometimes trembles when he talks about the war. But he will try to talk anyway because, he said, “it’s very important that they know about Vietnam and the sacrifices that were made.”