I got my TB mare in Jan 2010 as a 3 year old, although she was not actually 3 until the end of May and she was 15.1hh and bum high. She is now technically 6 (although not quite yet!) and has just levelled out at....................17hh!!! So yes!!
I think the concensus is that the skeleton doesn't fully mature until 6/7 (the end of the teenage years) so easily possible. Clearly depends on her parents and good nutrition to make the best of her genetics
Yes they certainly do grow.
I owned three TB's in the past. My first one was a very finely built 3 year old when I got him and he grew 2" to 16.1 when he was 5 years old.
My next one was a 7 year old heavy set 16.2 and had already finished growing by the time I got him.
My last boy was 5 1/2 years old, in extremely poor condition, skin and bones and at least 150kg underweight. My vet at the time told me he wouldn't grow any more but would hopefully fill out with the right nutrition. Hmmm, he was wrong!
Much to everyones surprise, and after lots of good feeding, TLC and putting him in work he shot up another 2 1/2" reaching his final height of 17.2 1/2hh by the age of 7.
I really think that nutrition certainly makes the difference.
One of the girls at our livery bought a 5 year old 16.1hh about a year ago as he was the height she wanted (she too was told he wouldn't grow any taller). He has grown 3 inches in the last 12 months and is still a little bum high so she thinks she might end up with him over 17hh.
Ours is just turned 5 and a very lightweight specimen with a big bum. Am very pleased to read of all these late developers as she looks bum high and immature and we are hoping for an inch or two more hopefully. She is definitely going through a gawky stage at the moment!
I got mine having just turned 3, he's now 6. He has grown 3 inches, I was just looking at his old girth, it wouldn't fit half of him now lol. He's flat bred.