My question is simple. There have been a lot of variations over the years between editions and dragon articles, but how many age categories do you think dragons ought to have?
Why should age need to be a consideration?
It shouldn't need to be. It's color attached to a fun mechanic for generating hit points to provide a rationalization. It can (and should) be replaced with any other reason that makes as much or more sense to the one doing the replacing.
In AD&D, all monsters (except "normal men" and their ilk) have d8's for hit dice, including dragons. For most monsters, every hit point is rolled-up on a d8, and by seeing the result of each roll, or even just hearing the number or general quantity of d8's rolled, the players can know or estimate the hit points of a potential adversary. Dragons are different. You don't know what you're going to get with a dragon because the DM only rolls two d8's and calculates the number of hit points in his or her head. The second d8 determines the dragon's "growth stage", or the number of hit points per hit die, so the number of stages is naturally 8. I believe this innovation was introduced into the game to make it more challenging.
A group that rolls monster hit points at the table could use this method no matter what edition of D&D it's playing. Depending on the size of hit die the dragon uses, the dragon's hit points can be determined by rolling a single die of that type and multiplying the result by the dragon's number of hit dice. This method greatly increases the possibility of encountering a dragons that is either much weaker or much stronger than the normal method would likely yield, and unless the players have memorized the number of hit dice had by each type of dragon, they won't be able to tell which unless they decide to find out by fighting it.
For example, in AD&D, 8 is the perfect number of growth stages due to the use of d8 for all dragon hit dice and the "growth stage" fluff-explanation for the mechanic works just fine, but in 5e (another edition I'm somewhat familiar with) dragons use hit dice from d8 to d20 depending on which size category the dragon fits into. To complicate things, the size category of the dragon is given a coloring rationalization corresponding to the "age" of the dragon, but let's ignore that for now. Each size category has a separate stat-block and can be essentially treated as a separate monster. Thus a dragon wyrmling could be considered to have 8 such "stages", a young dragon 10, an adult dragon 12, and an ancient dragon 20, making for a whopping 50 "stages" for each dragon species. Obviously "growth stage" or age of the dragon no longer serves to explain the mechanic in this case, but relative health or some other colorful explanation could easily be substituted.
To summarize, if your group uses average hit points per hit die or hit points assigned (not rolled) by the DM, then the concept of "growth stage" may not be useful and might as well be assigned to an individual dragon at the DM's whim just like any other element of colorful description. But if your group likes to roll monster hit points a the table and would like to preserve the unique play-experience that dragons had in earlier editions of the game, then rolling a single hit die might be a fun alternative to the norm.