When you are able to acquire a new familiar, you may select a wyrmling dragon as a familiar.
Prerequisites: Cha 13, Speak Language (Draconic), arcane spellcaster level 7th, ability to acquire a new familiar, compatible alignment.
Benefit: When acquiring a new familiar, you can choose a wyrmling dragon. See Dragons as Familiars (Dr 141) for more information.
Source: Draconomicon, page 104.