Where’d lemmy go? The stage is empty: vacated mics, cooling drum stool, the blocky, buzzing statuary of amps and speakers. Motörhead, the legendary Motörhead, is not there anymore. I’m in a heavy-metal hangar in Salt Lake City in late August, and singer/bassist Ian Fraser “Lemmy” Kilmister has just walked off, shakily and in evident distress, after only four songs, anxiously pursued by his drummer, Mikkey Dee, and guitarist, Phil Campbell. A man in a bandanna approaches me, pop-eyed with dire foreknowledge: “He’s not comin’ back, man! He’s not comin’ back! He’s too old!” Then he reels away, into the hormonal half-smoke and press of bodies in front of the stage. Should we riot? Are we sad? Is it possible that Lemmy—69 years old, pacemakered, diabetic—Lemmy, the great survivor, opposer, grizzled odds-beater, humanity’s middle finger, was crying? “Listen,” he’d said to us before exiting, in his familiar English roar-gasp, that voice of fiery exhaustion. “I’m really sorry—I can’t tell you how sorry I am—but my back’s gone. I’ve got this bad back and … I can’t breathe up here either.” Then he covered his face with his hands, and he left us.

Now what? Ear-hum, and slitherings of suddenly surplus electricity. People are milling around pre-violently; a scuffle breaks out to my left, a centripetal skirmish sucking in bouncers, and the smell of snuffed adrenaline rises. Diehards are chanting: “Lem-my! Lem-my!” But Lemmy’s not comin’ back, man. He’s too old. It’s time to go, time to get out, into the Salt Lake City night, where the bike engines phlegmily rumble and the sprinklers complacently hiss. What if—a numb little thought, bubble-like in the desert air—what if this was the last Motörhead show?

There is a second prime, we are discovering, in the life cycle of a rock-and-roller, a madder and more precarious second heyday. The potency of early manhood passes, and its beauty is a memory. Barely a blip now travels around the once-blazing circuit of your inspiration. Your bones ache, your voice is shot, and the rags of age are upon you. But you keep going. You keep playing. And gradually this becomes the thing about you: You’re still there. You endure, you defy, and the older and gnarlier you get, the more magnificent the rebellion is. Creaking recklessly, in swaggering infirmity, you sally forth; you hit the road again and again (and again) and you give the people what they want. And now, check it out, they don’t just worship you. Now they love you.

In hard rock and heavy metal, of course, this dynamic and its attendant pathos are magnified, because in hard rock and heavy metal everything is magnified. Voices are distorted, amps are overdriven, performance is an onslaught. Volume projects power: A scream or chord, inhumanly sustained, outfaces mortality. And then there’s the lifestyle, a long-term test of the capacities. Some droop, some drop, but the music never subsides. Slayer just released Repentless, its first album without guitarist Jeff Hanneman, who died in 2013 of alcohol-related cirrhosis; now all of the band’s songs are written by Hanneman’s co-maniac, Kerry King. Iron Maiden has a new album out, too. Bruce Dickinson recorded the (characteristically soaring and outrageous) vocals with an undiagnosed tumor on his tongue. (“[They] took a scan of it,” he told the BBC , “and went, ‘You have head and neck cancer.’ So I went, ‘That’s a bit of a blow.’ ”)

A week before the Motörhead show in Utah, my friend John and I drive out to Gillette Stadium, home of the New England Patriots, to see AC/DC. We make a fragile pair in John’s Mini Cooper: me, nibbling fennel seeds to soothe a grumbling tummy; him, munching lorazepam for the management of big-crowd anxiety. But the promise of AC/DC, of massive, major-chord resolution, shines before us like the promise of health. This is the music of ragers and rallies and bombing runs, of elemental affirmation and destruction. Like Motörhead, AC/DC comes out of the ’70s, when hard rock was gearing up for the domination it would enjoy in the ’80s; like Motörhead, AC/DC has hardly changed, riding out every subsequent fashion and fragmentation. Now, I tell John, now is the time to see AC/DC—not in 1977, in the band’s hooligan pomp, when its split-the-atom Australian blues was so fresh as to be almost avant-garde, but tonight, in 2015, in its wild senescence. We pay $25 to park three-quarters of a mile away from the stadium and continue on foot, passing a man dressed in the folkloric costume of lead guitarist Angus Young: schoolboy cap, blazer, and shorts. He wields a cardboard guitar.

For AC/DC, too, it’s been a rugged season. Last year it was announced that rhythm guitarist Malcolm Young, 62, Angus’s brother, was suffering from dementia and could no longer continue in the band. Grotesque irony, that Malcolm, author of the most unforgettable riffs in rock and roll, should now be unable to remember them. Then Phil Rudd, the drummer whose disco-pistoned simplicity drove AC/DC from 1975 to 1983 and again from 1994 to 2014, was arrested last November and later convicted of drug possession and threatening to kill a former employee. Chris Slade is now on the drums, while Malcolm’s slot has been filled by his 58-year-old nephew, Stevie. (The Youngs are a clannish crew.) Stevie Young fits right in, presenting next to Angus a spectacle of withered and slightly vicious consanguinity. But Malcolm is irreplaceable: the huge, benign tensions he summoned on the fretboard of his Gretsch, the anti-chords called into being by his chopped super-chords, his grimly joyful face and grimly twitching body. Then again, replacing the irreplaceable is what AC/DC does. When Bon Scott, the original singer, died in 1980 at the age of 33, after choking on his own vomit in a parked car, it took the band mere weeks to hire Brian Johnson and start recording Back in Black.

As dusk falls and the air cools, the great bowl of the Gillette scintillates with the restless, insectile blink-blink of a thousand pairs of toy devil horns—tiny red lights, everywhere. You can get your horns at the concession stand, all part of AC/DC’s jolly postmodern diabolism: “Hell Ain’t a Bad Place to Be,” etc. And there on the big screen is Angus, with his pale, bare shins and his gibbering kneecaps and his head going up and down, up and down, in a contemplative frenzy, a 60-year-old man dressed like a schoolboy. He strikes his black Gibson, right arm lifting away from it in tribute. It takes a second for the sound to reach us, and in that time lag is the span of our adoration. This is the scalar difference between Motörhead and AC/DC: Lemmy has always spoken profoundly and poetically to his constituency of banged-up bikers and disaffectees; Angus is global.

But when it’s done—and it’s almost done—there will be no more Anguses, no more Lemmys. The bloody-minded, death-demolishing longevity of AC/DC and Motörhead cannot be counterfeited or repeated. Lemmy once roadie’d for Jimi Hendrix; these days, retiring postshow to his tour-bus bunk, he reads P. G. Wodehouse. His mic stand, for 40 years, has been higher than his head, microphone angled downward, the better to catch the heavy-metal plasma shooting from his larynx. And now he’s disappearing into the dark wings of the stage, taking with him his grave-digger wit and his gnashing bass and the gorgeous, ruinous momentum of his music.

You’re not supposed to go gentle into Dylan Thomas’s good night. Rage, rage, and all that. But it makes you gentle, the going. It takes your strength. The night after Salt Lake City, Motörhead canceled a show in Denver. In Austin, four days later, Lemmy left the stage after three songs. Lem-my! Lem‑my! I can hear the crowd, the lowing, forsaken mob, letting off steam and honoring his frailty. In the middle of all this, Motörhead’s new album, Bad Magic, was released. There’s a track on it called “Thunder and Lightning”: Stand on the stage, promises made / Under the blade, scratching and biting. How hard it must be, and what strange grandeur in the effort, to try to keep those promises. To stay standing, even as the blade comes down.