Going off of only having played 3.5, 4e and 5 with any real regularity, I'm not particularly happy with any of them.
There should I think be:
Whelp (because baby dragons are cute!)
Juvenile
Young
Adult
Mature
Ancient
(Legendary)
I throw in "legendary" more as "this is where you write the really cool, really powerful special snowflake dragons". It's not a category that every species of dragon needs to actually have a stat block for, but more of a conceptual meta location for how much more powerful than "ancient" special dragons should be.
So, that category aside, 6 seems reasonable.
Ok, cool. So vote if you are participating!
Why should age need to be a consideration? Is an orc older because it has 8 hit points instead of 4? Or is it just a bit bigger/tougher? Age should not need to be tied directly to hit dice/hit points per die or the like for dragons. Every other critter in the book does not make it a factor.
As an Aside: I've never liked how different dragon types have a clear statistical concordance, within each age category. Reds rule, blues are cool, greens clean, blacks lack, whites drool.I suppose from a DM perspective it makes it more likely you can find a just-so level/CR dragon, assuming you can get the party to the appropriate terrain type -- but it just offends my sense of symmetry.
According to the AD&D 2nd Edition Monstrous Manual, 1993 p64.
01. Hatchling 0-5 years
02. Very Young 6-15 years
03. Young 16-25 years
04. Juvenile 26-50 years
05. Young Adult 51-100 years
06. Adult 101-200 years
07. Mature Adult 201-400 years
08. Old 401-600 years
09. Very Old 604-800 years
10. Venerable 801-1,000 years
11. Wyrm 1,001-1,200 years
12. Great Wyrn 1,200+
Each age catagory has their own modifiers, and each dragon type has their own modifications too (body length, tail length, AC, Breath Weapon damage, Magic Resistance, treasure type, XP Value).
As for how later editions breaks this down, I do not know.