The Air Dragon and its hybrids become available at Level 14 (as do the Water Dragons).
The Air Dragon is somewhat unusual in that it's both a basic elemental type used to create other dragons with the 'Air' element but it can also be generated rather than bought by breeding two dragons with the 'Fire' and 'Water' elements. Breeding two air dragons to breed another air dragon for sale is an excellent way to generate money quickly as you will generate 500,000 coins every 4 hours. Metal Dragon hybrids provide even better resale value but don't appear until Level 17.
"Air dragons spend almost their entire lives in the air -- eating, playing and even sleeping without ever touching the ground. It is rare indeed that anyone ever gets to view this dragon up close. Wizards have confirmed that this dragon is the result of breeding fire and water dragons."