Singer-songwriter and actor Kris Kristofferson. (Courtesy of Kris Kristofferson)

The premise — and the promise — of country music is that it tells the truth. That must be why so many country singers tour so doggedly into the twilight, even when their bodies are hurting and their minds are tired. They feel a responsibility to tell the biggest truth there is: Life is valuable because it ends.

It’s a lesson you might have picked up from Kris Kristofferson’s music way back in 1970, when the legendary songwriter released his astonishingly sage debut album at the age of 33. But when Kristofferson, now 80, sang many of those very ballads at Washington’s Warner Theatre on Thursday night, there was a fresh urgency on his tired breath. Kristofferson continues to battle memory loss and Lyme disease (initially misdiagnosed as Alzheimer’s), and his current tour has reportedly been dotted with foggy, uneven performances. Thursday night’s concert, however, went off without a hitch, perhaps because it was so simple — a man standing alone onstage with his guitar, his harmonica and some of the most illuminating country songs ever penned.

There’s no denying that Kristofferson’s voice has coarsened in his old age — enough to make you wonder what really happens to our airways after 80. Do they become knotted, cragged, rumpled and elongated? Every lyric the man sang sounded as if it had inordinate ground to cover as it traveled from his lungs to his lips, and the effort made him sound both weary and heroic. When his voice plunged to its roughest, lowest place during “A Moment of Forever” — “I’m so glad that I was close to you for a moment of forever” — he made enlightenment sound like something that must be earned.

It was one of many songs where Kristofferson had etched a fleeting moment into eternity. There was the desirous flicker-flame of “Help Me Make It Through the Night,” and the seize-the-day courage of “I’d Rather Be Sorry,” and as he moved swiftly from one tune to the next, keeping his banter brief, Kristofferson seemed to be speaking directly to the audience through his lyrics — something that clicked during “For the Good Times” when he sang, “Let’s just be glad we had some time to spend together.” The room couldn’t stop itself from bursting into full applause.

Throughout the night, the slightly halting feel of Kristofferson’s guitar picking seemed to match the tone of his voice, but during 2013’s “Feeling Mortal,” he addressed higher powers in an unimpeded bellow, perhaps in hopes of being heard: “God, Almighty, I’m here/Am I where I oughta be?” The lyrical rejoinder — “I’ve begun to soon descend/like the sun into the sea” — came softer, but there was no ambiguity to this stunning moment. He was putting his house in order, in song.