Talk of how to create a dragon in Swords & Wizardry inspired me to put this together.
First, Dragons are ridiculously dangerous opponents. They're intelligent, shaped for mayhem and murder, long-lived, possessed of enormous amounts of resources which they'll happily use to hire someone to kill you.
So, things like HD, attacks, breath damage, etc. aren't super instructive when differentiating between Dragon ages (for me). They're also kind of boring; the granularity is interesting, but much less so if we're just talking about a change in a few mechanics. None of the below has to be true at all, it's just an ecology of Dragons based pretty much solely on the Dragon section in S&W Whitebox (free download of the whole thing here).
HOW BIG ARE THEY? HOW OLD?
Standard S&W rules apply for HD and damage, so roll a d8 for age:
1. Very Young Dragons (years 0-3 or so)
"Hatchlings," about 5 feet long from end to end, lives in a brood of 3-18, flies at half speed and is ungainly in the air. Highly intelligent hunters, will likely lure prey into a trap, when solitary, will fight only when cornered, otherwise, will prefer to nip/breathe/claw and run until its full numbers are available at which time all will pounce on the weakest looking target. Highly incautious and may not necessarily view adventurers as threats, more as playthings (they still kill their playthings). Most will hunt nearby wildlife or small humanoids. A forest devoid of game is often an early sign of a nearby brood. Most will nest near where they were born as they tend to be born in locations well-hidden.
2. Young (years 4-5 or so)
"Whelps," about ten feet long from end to end, lives in a brood of 3-12. Will fight as hatchlings, will also have hunting patterns more established, will be highly aware of the terrain and may have more elaborate traps established. By this point some of its brood have likely died to illness, predation, roughhousing or adventurers and the Dragon will be much less eager to engage in melee if numbers aren't on its side, prefer to flee and fight another day (or more likely, return with the rest of its brood and stalk its former attacker). Will begin to hunt and stalk larger, more intelligent creatures, especially orc and other humanoids living along the fringes of civilization. Eat less frequently than younger Dragon but tend to glut themselves when they do, mimicking the feasting/hibernation pattern of older Dragon.
3. Immature (years 6-10 or so)
"Hunters," about fifteen feet from end to end, live in groups of 3-9. Dragons of this age will still often hunt and live in a group, but as they age, they begin to feel a certain distrust of others of their kin and the first roots of their greed and covetousness take hold. They will develop a craving for privacy and independence from the brood, but still prepare these lairs with the help of the brood, carving out a network of caves, mazes and traps replete with murderholes and the like which take maximum advantage of their breath weapon and other natural abilities. The obsession with preparing the most defensible lair is highly ingrained, part nesting behavior, part a coping strategy for leaving the safety of the brood's numbers.
Most will lair where their nature inclines or where they are best adapted (white dragons in areas with lots of snow, black dragons in swamps, blue dragons in abandoned, lightning-blasted wizard's towers), but this is far from a necessity. Immature Dragons are highly proud of their lairs and the various traps therein and the brood will compete and obsess over these lairs and increasingly elaborate tricks and traps.
Immature Dragons intentionally hunt and bait intelligent humanoids and pride themselves on their ability to take down increasingly dangerous prey. Most will intentionally hunt away from their lair and will bring food home for later. Some may even store food away from the brood, to prevent discovery of their siblings.
4. Adult (years 11-60 or so)
"Lairing," about 20 feet from end to end. May live with one other of its kind, if only to have someone around to defend the lair while you're out foraging through towns and villages for food. Dragons of this age are frequent and wide-ranging hunters. As they begin to acquire more material goods, whether from conquests or as the result of defending its lair from looters, a Lairing Dragon will become slowly covetous and jealous, increasingly solitary and acquisitive. It's rare that any Dragon of this age that his laired with another will make it to old age without killing its partner in a fit of jealous, mistrust or abject fury.
5. Old (years 61-99 or so)
"Solitary," about 20 feet from end to end. At this point, the Dragon is truly greed and solitary. If any of its brood or a Lairing partner have thus far survived, the Solitary Dragon is obsessed with finding them and silencing them and accordingly, Dragons this old are likely to die by the hand of one another than looters. Dragons of this age rarely hunt or even leave their lair but when they do, the result is often catastrophic for the surrounding area. After a good meal, the Dragon will sleep for days.
6. Very Old (years 100-200 or so)
"Hibernating," about 20-40 feet from end to end, depending on precise age. Hibernating Dragons are rarely awake, but will decimate a series of towns or even an ill-prepared city when hungry. Most sleep for years and spend their little waking time not dedicated to eating to shoring up their traps and burying their hoard ever deeper. As they age, their waking/sleeping cycle is increasingly associated less with hunger and more with nearby cataclysmic events, making their awakening fortuitous (waking the day of an enormous battle, the winning side easy pickings, waking on the day of a high holiday in a nearby city, when the revelers are least cautious). The Dragons hoard likely contains at least one truly unique magical item, likely something from another age, perhaps an intelligent item. This will undoubtedly be the Dragon's prized possession and it may risk all to get it back.
During hibernation, particularly the last, long sleep it undergoes at around age 195 or so, the Dragon will begin to change, grow larger, grow more horns, eyes, wings, may take the ability to shapechange, may be able to cast magic as a Magic-User of its HD. It's scales become thicker, hollow.
7. Aged (years 201-399 or so)
"Chiming," about 60 feet from end to end. The scales of Chiming Dragons are hollow and they produce alien tones when they move. These tones can be addictive, hearing them can be similar to taking psychoactive drugs (save v paralysis or you're tripping) but will grant visions of strange cosmological significance. Dragon addicts/cultists will likely attend/worship the Dragon. Dragons no longer sleep and rarely eat. When they do eat, they eat any and everything: rocks, sound, trees, the living, the dead, shadows, thoughts. Chiming Dragons begin to care less for their material hoard and more for lost knowledge, secrets. They may well kidnap religious leaders, powerful wizards in hopes of teasing out secrets from them (or attracting the attention of those even more powerful). May trade quite a lot of gold for a good secret (but then may try to kill you as they prefer their secrets...secret).
Near the end of this period, the Dragon will begin to build hundreds of cocoons thickly walled by its dropped, chiming scales.
8. Ancient (400+ years old)
"True," about 80 feet from end to end (or much, much larger). Most True Dragons take on a different shape as their original is often painfully arthritic. True Dragons likely grant cleric spells as any other divine being. They now eat only strange, intangible things and they burn for the secrets of the universe. True Dragons may live for thousands of years before their memory begins to fail. This is the Dragon's greatest fear and many will do whatever they can, including inciting their worshipers into single-minded pursuit of a means of halting the progress of dementia and memory loss. Those Dragons that fail (it's believed that all have) commit ritual suicide, vomiting all of their secrets into the cocoon left in their lair. The cocoons will hatch into other Dragons or perhaps things stranger still, depending on the secret.
HOW DO THEY FIGHT?
Dragons are highly intelligent and clever but they fight like animals. They will try to bullrush, headbutt, use their tail and the length of their body to knock down and crush before bringing tooth and claw to bear. Larger Dragons will likely just try and step on you.
THINGS THE DRAGONS KNOW
Dragons are pathologically acquisitive and this behavior extends to less material goods as well. They collect and covet information (especially secrets), spellbooks and other sorts of less immediately tangible power. Accordingly, a Dragon always knows secrets about the area surrounding its lair. If the elf fortress has a secret backdoor, the Dragon knows it. If there is a cure for the plague ravaging the nearby towns, the Dragon knows it. How you get that information out of the Dragon is another matter.
Adult and Old Dragons know one of the things below. Very Old know two things, Aged know three and Ancient know three plus something apocalyptic.
the secret of a star
the recipes necessary to cook star flesh
the weakness of a god
why the elves do not die
what dwarves fear most
the language of an old, lost race
the location of something a character most wants/needs
the location and key to the treasure vault of another Dragon (or God)
the true name of another Dragon
a book containing the most powerful spell (ritual) the Dragon knows
what is buried beneath the city of the overlord
the magical sentence that will cause the giants to curl up like dead spiders
how to learn the secrets of trees or stones or the wind or rain
the secret of crow boy
what happened to the leader of your religion
where they've stashed their hoard
how to eat time
how to swallow sound
WHAT AN ANCIENT DRAGON KNOWS (APOCALYPTIC STUFF)
this should include stuff that makes sense in the context of your setting, like:
the location of one of the pillars that holds up the world
the true name of the worm that encircles the universe and is slowly devouring itself. When sated, it will crush the world between two hoary talons
a ritual that remakes reality
the location of the forge in which the world was fashioned
pictures: Albín Brunovský, Brian Froud