As a dragon ages, it grows larger and gains more magical influence. It starts roughly human sized when hatching from the egg, and rapidly grows to dozens of feet long and tall within ten years. The rate of growth tapers off after this, the fastest dragons growing at a foot in length and several inches in height every year for the rest of their lives.
The natural lifespan of a dragon is not known. While still living on Earth, dragons were always killed by humans when they got to a certain size, and during the Draconic Schism War, dragons were killed off faster than they could breed. Because of this, it is only now that the limits of dragon biology are being pushed with certain individuals, but it is impossible to say for certain which dragon is currently the oldest.
When a dragon reaches roughly 500 feet tall, depending on the individual, it becomes impossible for the biology to support itself. Past this point, the dragon must then use its' command of magic in order to stay in that form comfortably. Eventually, there is a second size point where the dragon cannot sustain its' natural form indefinitely - here, the dragon must assume other forms in order to survive. This form can be anything small enough for them to support the new size with their magic, except a smaller dragon form; a dragon can only assume a draconic form of their own dragon biology, and must use illusion spells to appear otherwise.
When the dragon chooses to assume the form of a weapon, it is then a Dragon Weapon.