The majority of your dragon's growing is done in the first 12 months, although they do continue to grow until about 18 months old. After that any growth they do is very slow, pretty much unnoticeable.
They pretty much go through growth spurt until 18 months old and after the growth rate slow down, but reptiles grow throughout their lives...just a tiny bit a time. I know a 10 year old Cyclura, a type of rock iguana from the Caribean, that was still growing. Last year he weigh less and was shorter SVL, but this year at 10 years he surprised his owner by growing another inch or so and added some pounds.
In my experience, they have growth spurts. Axl seemed like he was never getting any bigger then out of the blue he got bigger. Now I'm waiting for him to grow again! They are so cute when they are small though!