In defiance of his shinigami side, Ichigo ages like a human.
It wouldn't bother Ikkaku, because he's seen plenty of humans age and die, except that this is Ichigo and he's never made friends with a human before. Strong ones in particular seem to waste away more dramatically, their years piling on like weights in brittle bones and painful joints.
Which isn't to say that Ichigo wasted away, because he's always been too strong for that. He ages from a freakishly strong teenager into a freakishly stronger man and from there into a still-freakishly-strong old geezer, bright-eyed and sprightly. But even that kind of spirit is no match for time, and by the time he's frail and sick, Rukia's stopped visiting (it's too painful, for her) and Renji's there less frequently (for him, too) though Ikkaku's never learned to leave well enough alone.
For a while, he'd seen the kid as his nephew- or maybe a bratty friend. When Ichigo reached his peak of strength in his thirties, they'd been 'brothers-in-arms', 'comrades', or maybe just plain 'brothers', as much as any of the men in his division. And then Ichigo had continued to age and Ikkaku had stayed the same.
(It probably says something about them, that they'd thrown around the idea of Ikkaku offing Ichigo early just so he could skip the aging and head straight for Soul Society but by then he'd had a wife, kids, so it never progressed beyond teasing. Ikkaku's the only person he ever made those jokes to, half wishful thinking, half nostalgia for the days he didn't have obligations like a job or a family, though he'd looked happier and more content then than any other time in his life. Probably because Ikkaku's the only person he knew who'd do it without hesitation if asked, even though he never did.)
Ichigo asks him, with the shadow of death hanging over his head and Ikkaku almost giddy with anticipation (his division had called first dibs on Ichigo decades ago), how he had died.
As a rule, the two of them don't talk about personal things, or serious things. Ikkaku's comments telling him to hurry up and die already would make anyone else intensely uneasy, but it had become a comfort, to Ichigo. People are waiting for me. I still have things to look forward to. I'll see my friends again. I'll be young again, healthy again, whole again.
It occurs to Ikkaku then that despite his bravado, Ichigo isn't so above his fear of death after all, his humanity and its will to live, to survive still strong even at this age. He sits on the table, one ankle crossed over the opposite knee, as arrogant, reckless and brimming with strength as the day they'd met. "Drowned," he says simply, and deflects Ichigo's next question, because of coursehe's a strong swimmer, he's always been, "there was a storm, the mast fell over and I got tangled in the ropes. Lent my knife to someone else, couldn't get through it with my teeth before I blacked out."
Ichigo can imagine it, can see in his mind's eye a younger Ikkaku falling into the water, thick ropes tangled around his limbs and him reaching for the blade he'd always kept at his hip, eyes widening in panic as he realizes that it isn't there. It's no wonder he rarely takes his hand off Hoozukimaru. It's a strange fear to have, because Ichigo's never seen him hesitant to dive into choppy waves, but the moment he loses his grasp on his sword, he's immediately uneasy.
"Sorry," he says, but Ikkaku dismisses him with a wave and a grin.
"Didn't have much to live for anyway." A brief silence and then, "Not like you, right?"
"Says the dumbass who's always telling me to die."
"Right, right. You've had a long, full life, old man."
"It doesn't feel like enough, compared to you guys."
Ikkaku snorts, pokes Ichigo lightly on the chest and says with a fair amount of pride, "No shit. Everything you've done, you had to squeeze into one lifetime. Me, I've had four, maybe five. Now stop talkin' like an old man and die already."
Ichigo chuckles, lets his head drop back against the pillow and closes his eyes. The time is 2:11 in the morning when he goes to sleep and it's 3:47 (Ikkaku had been staring at the digital clock on the wall- what? it's not like there's anything else to do) when he decidedly won't wake up again.
He's taking his sweet time, though, so Ikkaku approaches, hand passing through Ichigo's until it catches on something solid and warm, pulsing with energy and he grabs it, pulling his spirit out of his body by the wrist.
Ichigo's smiling at the sudden weightlessness, but his face falls when he looks at his hand, at the wrinkles and liver spots still on it. Ikkaku doesn't say anything, only draws Hoozukimaru and cleanly severs his chain, grinning at the wonder in Ichigo's eyes as his back unbows, his skin tightens and he's back to the way he was at seventeen. He's weaker now than he was then, but there's a predatory glint in Ikkaku's eye that says he'll have plenty of time to get back to his prime.
Before Ichigo can protest, Ikkaku's bumping the bottom of Hoozukimaru's hilt on his forehead, much harder than strictly necessary because it's been a long time since he could be rough with Ichigo without fear of hurting him.
"Hey-"
"I'll see you on the other side, idiot."