My kiddo was diagnosed at 16. She was 5'1.5 when diagnosed and did grow half an inch after being diagnosed and treated, but she was pretty much done growing at that point. She was always going to be small though, due to genetics.
However, each kid is different. Kids who have had the disease for a long time without being diagnosed often have a delayed bone age. They check that by taking an x-ray and look to see if the growth plates have closed and what age the bones show, if that makes sense.
That means, for example, if a child is 12 years old but has had active disease for quite a while, his bone growth might be slower and his bone age may show that of a 10 year old. So his bone age would be 2 years behind his actual age.
So, if your bone age is delayed, you have more time to "catch up" with your peers. The best thing to do to help you catch up, is making sure your disease is controlled and you're getting enough calories. Some kids do need to have a formula supplement (such as Boost or Ensure or Peptamen) to get additional calories/nutrition.
My daughter was on formula for several years and it really helped her get to a normal weight - she was severely underweight and malnourished at that point. She went from 80 lbs at her worst, to a healthy weight of 110 lbs with the shakes. She also said she just felt much better - had much more energy and just felt better.
Hang in there - things WILL get better.