I think he is growing, its just very slowly as people age and have physical growth different than in the human world.
For example - rukia, shes near 100 years old, but looks like a teen.
hitsuguya will likely get a couple inches taller as he enters a more physcial adult form but that could take 50-100 years from where we are now if we guess 10-1 ratio for age vs physical development.
Toshiro is slightly taller then Gin was when he joined Aizens squad. 100 years later and Gin looked all grown.
Not saying that all shinigamis grow at the same phase but it looks like a slower natural progression.
He will become taller....and more awesome.
I'm not even sure how the aging process works in SS... Gin aged into an adult within a century, and Kyoraku went from a pre-pubescent kid to a man in his 30's or 40's over a thousand years.
Dunno, I am somewhat of the idea that for shinigami growing is much like for humans. Some people reach puberty at 11 while others get there well past 14... While it accounts for a couple of years in real life such a thing can represent hundreds of years for a shinigami. Hiyori is the main cause for confusion in this particular area (she appears to be older than byakuya and gin however she remains young) however if we consider shinigami puberty came late for her it would perhaps explain what we saw (and a 110 year old puberty would more than explain her being a bitch). Realistically speaking it seems byakuya and gin are supposed to look like teenagers the age of ichigo or perhaps a tad older, just look what they looked like when they were introduced. In hitsugaya's case he seems to be in fact exceedingly young by shinigami standards, perhaps even younger than renji, rukia, hisagi or kira who are more or less of the same age.
There are few people we have actually seen grow in the series. Byakuya, gin, hisagi, nanao, ganju and matsumoto are the main examples. Byakuya was a teenager, hisagi was a little kid, gin was a bit younger than byakuya, and nanao seemed even younger than gin and matsumoto appearance wise appeared older than gin however that would indeed be impossible to confirm. Ganju was seen in a flashback from when kaien died as a little boy which supposedly happen some 40 years ago. Then again ganju is supposed to look ichigo's age....
Anyways, based on the few examples we have on people growing it seems than it takes around 50/60 and 100 years for a kid to take on an adult appearance. Byakuya appears essentially as he looks now in a flashback from when rukia was adopted. Gin appeared in an omake from hisagi's time in the academy and it seemed like at the time he looked much like what he looks now. If we consider ganju then perhaps the number could be smaller considering he became at least a teenager in about 40 years or so and he seemed specially small back then. Perhaps while hiyori could be an example of someone reaching puberty really late ganju could be an example of someone reaching it early (or perhaps he looked like a little kid for a long time and puberty hit him like a meteor).
Now, there is still the issue that we don't know just how old people were at any of this points. We know byakuya and gin are somewhat similar because ukitake and kaien discussed that (with gin being the younger one). We have no actual clue of how old hiyori actually is. She was a VC 110 years ago however we don't know if 50 years ago she looked like nanao did at the time. We don't know for how long byakuya looked like a teenager before TBPA arc either.... Still, I would think my idea is somewhat consistent and hiyori can be explained by a combination of being the sort of person who ages really well and reaching puberty late.
Well, i expect Hitsugaya to grow in a future time skip after this war.
I think people only grow in height during peaceful ages.
I'm expecting him to grow taller than Hinamori. At least.
Looking at Hiyori 100 years ago and now... sorry Hitsu, no growth for ya
kkck, can we just consider Toshiro & Hiyori (especially her) the dwarfs amongst the shinigamis?
Some people just keep their youthful appearance. Bad for someone who has to wield authority, nice for most others. By now it's pretty clear that it's not only puberty that holding him back in his growth, he's a short guy by genetics too, at least he has to by some degree. Same case with Hiyori, she seems to be more of a childish woman than a kid herself. I respect them as damn near everybody else in the series seems to be fucking gigantic in comparison. Even if the databook say something else, look at Kubo drawing Ichigo with that nonexistent mass, he's the prototype with his stick figure.
At least the Royal yal Guards allow him some variety in body types.
Maybe he has some type of growth defect. Shinigami don't seem to be invulnerable to human illnesses and conditions.
As someone else said, Hiyori didn't change that much, so I guess that Hitsu won't grow taller.
Just a thought on how it might make the aging different. But most likely Kubo hasn't though ot the aging to the extent we have :P
I am not against the idea of reiatsu being related to aging however what we have seen of hitsugaya does not actually support the idea. Hitsugaya is extremely young by any standard, too young for us to be able to suggest that it is his reiatsu is what is keeping him young. If hitsugaya was 300 years old we would actually see something in comparison abnormal to other shinigami however that is not the case, nothing suggests hitsugaya is even older than any of the current VCs. In context, this would be like seeing a normal 3 year old in terms of height and physical appearance and suggesting he is going to age slowly because he does not look 10 yet. If the same person was 40 but looked 20 then we have an entirely different situation. As things are we have no reason to suggest hitsugaya looks any different from what shinigami his age normally look.
Not to mention that the comparison with naruto is somewhat out of context. Shinigami all have shinigami reiatsu. Reiatsu being weird or special usually ends up pointing to a fighter being of another kind or specie altogether. In this regard all shinigami have precisely the same kind of shinigami reiatsu and the same can be said for fullbringers, humans, arrancar and vizards (although arrancar and vizards seem to have the same kind of reiatsu). In turn what you see in naruto is that while still human or ninjas you have humans with inborn genetic traits or chakra's innately and objectively better and stronger than that of ninjas unfortunate enough to not be born objectively better than normal people. Its not like training to get a strong chakra in naruto-verse can make you live longer, you have to be born as an objectively better human being with either senju or uchiha genes or at least be some sort of descendant of the rikudo.