Well, that depends on which type of dragon you would like to make, here is a small rundown of types:
'True' Dragons: 12 age categories, include among others the following: Red, Black, Gold, Brass, Brown, Shadow, Sapphire, Emerald, Celestial, Typhoon, Force and Prismatic Dragons.
Drakes: Small dragons similar to animals, different types vary in size from tiny to Huge, includes (Among others): Wyvern, Felldrakes
Linnorms: Large wingless dragons that are allways either chaotic or evil (Usually both), no lawful or good linnorms exist. They are thought to be almost extinct as no baby linnorms have been seen for hundreds of years. This type includes the Dread Linnorm and the Corpse Tearer
Wurms (I): Neutral (N, LN, CN, NG or NE) dragons aligned with nature, has four age categories.
Wurms (II): This group is comprised of primitive, legeless dragons, sometimes also called serpents.
Planar Dragons: Dragons from specific outer, inner or transitive planes (note that the shadow dragon is not a planar dragon), this group includes the Hellfire Wyrm.
Note that several of these groups of dragons have subgroups and subtypes.
Later,