How did Shunsui grew up so quickly?
The four images above shows Shunsui's growth throughout the series.
A kid,
Teenager,
Young Adult,
His default looks, Middle aged.
The major problem with this is that it creates a plot hole as there's barely any time difference between them.
"It is said that any man who joins the Ise Clan will die an early death, and this phenomenon is called the Ise Curse"
The information above shows that Shunsui's brother died shortly after his marriage, that short period could be a few years which is still low for Shinigamis.
It can also be assumed that there's enough time difference between 2nd & 3rd image because Nanao looks like she's 5-6 at least which could be decades for Shinigamis (but then there's Ichika who's not even 10 yet and has totally aged normally), however the main problem will arise with the 4th image because Nanao is still a kid in that and Shunsui has already aged a lot.
One approach is to assume Shunsui is almost of the same age in the 3rd & 4th image, it's just he simply looks old because of chest, facial hair and maybe some stressed life after becoming a captain.
Another is to assume that Nanao did not age for centuries as a child but this one is extremely unlikely.
However then there's an another problem that in the pendulum flashback Nanao still looks like a kid which only happened like 100 years ago.
In the first image, Yamamoto looks like how he looked during the war against Yhwach which happened 1000 years ago so I'm sure everyone would've thought that both Shunsui & Ukitake are closer to Yamamoto & Unohana than the rest, however the backstory of Shunsui with his sister-in-law makes both Shunsui & Ukitake closer to the age of Byakuya.
So the answer could simply be either of them:
Yamamoto just suddenly aged 20 years during the past 2 centuries;
Shunsui did not age for centuries as a kid;
Nanao did not age for centuries as a kid or was carried by her mother for decades or a century.
Shinigamis can decide whether to slowly age or age like how humans do or maybe it's just random.
Kubo did not care much about timelines.