All righty! Here are dragon ages with pictures to show approximate size.
Wyrmling: 0-5 years. Often referred to as Hatchlings in their first year.
Very Young: 6-15 years. Not much bigger than wyrmlings, but much more sure-footed and confident in themselves.
Young: 16-25 years. This is where they really begin growing towards their adult bodies.
Juvenile: 26-50 years. They are well on their way towards adulthood and generally begin to develop their magic at this point.
Young Adult: 51-100 years. They have now started to become formidable, their magic strong and scales hard enough to turn aside nearly all weaponry save magic and the teeth and claws of another dragon. They can also begin to mate at this age should they so choose to do so.
Adult: 101-200 years. Their growth has slowed, but they are just beginning to hit their prime.
Mature Adult: 201-600 years. While growth has slowed, they do not stop growing for a long time yet. Their magic ability and skills are continuously being honed. At this point, the dragon has often made a name for themselves.
Old: 601-800 years. Their scales nigh impenetrable, old dragons generally do not seek out battles of brawn, but battles of wits, taking more entertainment in pursuits of the mind and magic. However, this does not make them weak in the slightest bit...
Very Old: 801-1,000 years. Not much as changed in this stage, though they may begin to show a bit more wear and tear from time in the form of small chips in scales and cracked claws and teeth.
Ancient: 1,001 to 1,500 years. At this stage, female reproduction decreases with fewer eggs and less success rates. Reaching this stage is a major accomplishment that garners much respect.
Wyrm: 1,501 years to 2,000. Reaching the Wyrm stage of a dragon's life is a great accomplishment achieved by very few. It is here that they are at the peak of their powers--physical, mental, and magical--and fear very little.
Great Wyrm: 2,001+ years. Only two dragons have ever reached the age to earn the status of a Great Wyrm. They are often believed to be reincarnations of Dragon Gods. The first was He Who Creates The World, who the dragons believe formed the very earth from his own body.
The second was She Who Watches The World, who was a Seer that dragons believe sees all--past, present, future. She is said to not even be confined to the earth.