So horses haven’t been growing for the last couple of weeks, since the old app got shut off. I re-created the growth today, but honestly have no idea why I picked the formulas that I did way back when. 
So, now the formula is more simple, and also you can see stats on your horse’s page about their height.
Formula:
This figures out how many days old a horse will be once they start their peak (peaks last several months/years, so starting the peak won’t mean the horse is finished improving). Then it figures out the retroactive math to decide the dates when a horse will have grown an inch, relative to their birth height, peak age, and max height.
So, for example, take a horse who is born at 9h. Their max height is 16h, so they will grow 7h (28 inches) from birth → start of peak. If we assume that they’ll peak on the day they turn 3 (1,095 days old), their formula would be:
1095 / 28 = 39.1 days per grown inch (rounded to 40)
So once they hit 40 days old, they’ll be 9.1h
Once they hit 80 days old, they’ll be 9.2h
Once they hit 120 days old, they’ll be 9.3h
…etc…
Once they hit 1,095 days old, they’ll be 16h
All of these numbers will be displayed on the horse’s page, on their Stats tab.
In addition, if we assume that we have this stats table because we’ve assigned Joe the groom to go measure all our horses every single day, he’ll also flag when a horse has stopped growing. This will happen on the same timeline as the rest of the growth, but at the point when he’d measure next.
So in our example, on day 1,135 (40 days after the horse hit their max height), Groom Joe goes to measure and says “hm, there’s been no growth since last time”. At that point, the stats list will show “Fully Grown” because they were growing 1in every 40 days and it’s now been 40 days since they’ve grown an inch (and are thus at max height).
Yearling who is not fully grown
Note that their height last updated 3 weeks ago, and it appears to be updating every 6ish weeks. You could look at his parents to get a general idea of what his full height will be, and (if you want) do the math to figure out when he’ll hit his peak. But if we just go based on growing 2h in a year, and he’s probably going to be 16h-ish based on his parents, he’ll probably hit full height around age 3.
Older horse who is fully grown
She was growing an inch every 4-5 weeks, it looks like, so if this feature had been live back in 2024, the “Fully Grown” label wouldn’t show up from Feb 7, 2024 to mid-March 2024, but after that, Groom Joe would go measure her and find her still to be 15.3h and would slap on that label (virtually, of course).
Hopefully this is helpful. I think you’ll find all the non-peaked horses will have shrunk, because like I said, I have no idea what my previous formula was doing (despite it only being 10 or so lines long) But this one makes sense in my head, anyway. Hopefully it does for you as well.