I actually hear the question about whether nor reptiles will grow to the size of their enclosure quite frequently...that is a common myth. All reptiles will grow to the size they're meant to be, regardless of cage size. If you keep the cage small to try and keep your pet small... you'll just end up with a big, cramped reptile in a small cage!
They grow the most the first year or two of their lives. After that, they will continue to grow very slightly throughout their lives, but it will never be the big growth spurt exhibited as babies. Some bearded dragons will reach close to full size in less than a year, while others grow a little bit slower and may take up to 2 years to stop growing significantly.
-Jen