“Youth is a state of mind,” a poet once said — but, young in spirit though they are, the elderly artist couple at the center of this fly-on-the-wall documentary must confront the harsh reality that aging isn’t something the body can avoid. Jackie and Don Seiden — a yin-and-yang pair who describe themselves as “a mouse and a crocodile” — still argue and make up with all the fierce vitality of a couple half their age. They haven’t yet lapsed into living life through the rear-view mirror: both still actively make art, Don his sketches and Jackie her slideshows and found-object arrangements. They live in a creaking yet beautiful home, decorated exclusively in pastel colors; as Don puts it, they’ve “made a life that’s really unusual […] a life only [they] could’ve made.”

As his health issues — and the weakening of her ability to care for them — threaten the end of that 50-year-long chapter in their lives, the couple confront mortality and find it brings them holding ever tighter to one another. Their abiding mutual affection makes this documentary a moving portrait of enduring love, while their fiery intellectual verve gives it a sharp honesty that prevents it from ever lapsing into sentimentality.

When we meet Don, his pace of life has already slowed down, but Jackie is still astonishingly nimble for her age, scampering around in a way that makes it blindingly obvious who the “mouse” in the relationship is. Still, the toll of a long life lived fully has begun to show itself on her joints, something the 78-year-old Jackie is devastated by because she never really believed it would happen to her. A moving scene finds her in the sharp center of this painful realization: she marvels and despairs at the irony that she, an artist who focuses on disintegrating materials, is now having to contend with her own body undergoing the same process. This scene is one of a handful in So Late So Soon that really encapsulates the agony of its title, and it’s impossible not to take the raw profundity of it as a lesson to ourselves.