One of the most commonly misunderstood, misinterpreted, or just plain missed things in the canon is the question of what, exactly, is going on with Chisei.
Which is to say: why is his age in quotes, is he being serious when he said he died, and what is going on?
The short answer is
Approximately ten years ago, Chisei collapsed on stage because of phantometal erosion and Haruomi did a metal kiss mind-dive to save him. This is apparently not well documented; Iori talks about it as being somewhat apocryphal, and even Allen, Buraikan mega fanboy that he is, has never heard about it. (The fact that this is an urban legend, and that nobody seems to know why Buraikan disappeared, may indicate that this was not as successful as Iori claims.)
[source: VIBES Part 2]
CLUB Paradox's re-opening and the announcement of the Paradox Live starts the series, ten years after Buraikan's sudden disappearance. Buraikan have not been seen at all in the intervening ten years, so people are super skeptical of the idea that Buraikan are actually involved with this at all, even as we get to the end of Stage Battle.
[source: Opening Show tracks and the skepticism is again referenced in the Buraikan is Back feature]
Their first major in-person appearance, together, is in LIVE pt.4: Haruomi is revealed to be the old man who runs Raimentei, appearing much older than he should. Chisei, meanwhile, appears to have not aged a day in ten years when he appears.
Haruomi seems surprised to see Chisei, who says that it's "thanks to [cozmez] that I'm able to come out like this", and remarks that Haruomi has become an old ramen-shop owner "because I up and died and left you behind". He is telling the literal, direct truth. This is not hyperbole, something that doesn't translate well, or a half-truth.
[source: LIVE Part 4]
Buraikan "ZERO" has Chisei directly remarking that producing an illusion of Chisei on top of his own is taking a toll on his body before Haruomi cuts him off.
[source: Buraikan "ZERO"]
Chisei's age is generally left unstated in materials such as site profile and the official fanbooks. One piece of official merchandise (seen here) gives Chisei's canonical age as 28. If assumed that this is his current age, then this would have made his career path improbably compact; and indeed, this is indeed contradicted by the later fanbook timeline, which places him as not even starting his professional basketball career until age 19. As no correction has ever been issued for the merchandise, this can be taken as indicative.
Early "evidence" from early MVs, though both of these points have been somewhat muddied by later MVs:
Though subtle, Chisei lacked a reflection in several early MVs, most notably in Rap Guerrila Reload and BURAIKAN is Back.
The visuals for the MV for Buraikan's "Back In The Day", even disregarding the extremely overt ending, juxtapose Chisei with images of a lighter going out, a cigarette being burned out, and TV static cutting off, before becoming a lit lighter that causes a cigarette juxtaposed with Haruomi to burn.
Kuzuryu Chisei is dead.
The Chisei that we see interacting with the rest of the cast is a metal phantom, created by Haruomi (inspired by Kanata doing the same to create a phantom Nayuta) and based on Chisei as he was ten years ago at age 28.
Haruomi's aged appearance in the present is loosely implied to be the result of premature aging thanks to habitual phantometal use, and it is possible that his own younger appearance is also a phantom created for the purpose of public appearances/performances.
When Chisei appears in LIVE part 4, he is an illusion. Haruomi, unlike Kanata, appears to be aware that he is manifesting an illusion of a dead person, but is keeping the illusion up for his own undisclosed reasons.
We know from phantom Nayuta that metal phantoms are capable of acting independently, being physically separate from, and speaking indepenently from the person generating them, and are corporeal enough to manipulate physical objects (see: the Nayuta phantom in the LOVE dramas being able to go places Kanata isn't, physically interact with people and objects, et al). Haruomi is not directly or consciously controlling Chisei.
Whether you take the anime as canon or not is your own cross to bear. Many parts of the anime, especially (unfortunately) the parts from which we draw this information, are incompatible with the drama track canon, but if you choose to accept it as canon, then it resolves this ambiguity entirely.
The anime features a phantom Buraikan created by Alter Trigger for the purposes of the first Paradox Live; if we attempt to fold this information into the drama canon, we can assume that Haruomi has elected to continue to create the phantom Chisei on his own. Indeed, the special voice dramas included as a bonus from the full BD set confirms that Haruomi is the one creating the Chisei illusion and that that Haruomi was specifically inspired by Kanata's use of phantoms to create a Chisei phantom.
Visuals at the beginning of episode 11 seem to confirm that it was phantometal erosion that killed him. A line from the Alter Trigger scientist may seem to contradict this, thanks to the way the official subtitles say "the same way Yasha died" as he's firing a gun at Shura; but, based on the actual line in Japanese, I believe this is probably meant to mean "and you're going to die, too", and is not a remark on how Chisei died.
Again: unfortunately, there are many things about the anime that are incompatible with canon, but the Buraikan information does mostly track with and confirm what was hinted at in the drama tracks, because Buraikan are basically the only group that made it out of the anime without their story and characterization being put through a meat grinder, lol.