Dragons can continue to grow for about 2 years, I wouldn't worry about it.
It used to be very common for dragons to not reach full size until 1 and a half to 2 years old.
There has been a major shift in the past 5 years or so in regards to feeding advice and the push to stuff these little growing dragons with as much food as they can. The result is a dragon who is basically full size by 8 months old... the consequences are arguably weaker health in the years ahead.
Provide a healthy diet and a healthy environment and your dragon will be healthy. Size and growth will come naturally.