Perhaps it is time to restate the part in question from the Draconomicon:
THE DIFFERENT KINDS OF DRAGONS
In the D&D game, the term “dragon” encompasses a number
of different creatures, some of which bear little resemblance to
the great flying creatures with breath weapons that we commonly
think of as dragons.
For the most part, this book concerns itself with the ten
varieties of true dragon described in the Monster Manual—
the five chromatic dragons (black, blue, green, red, white) and
the five metallic dragons (brass, bronze, copper, gold, silver).
True dragons are those creatures that become more powerful
as they grow older.
A number of other true dragons are described in Chapter 4
of this book. In addition, Appendix 2: Index of Dragons provides
a complete list of all true dragons that have been presented in
official sources.
Other creatures of the dragon type that do not advance
through age categories are referred to as lesser dragons (which
should not be taken to mean that they are necessarily less
formidable than true dragons).
The three kinds of lesser dragon described in the Monster
Manual are the dragon turtle, the pseudodragon, and the
wyvern. Chapter 4 of this book contains a number of descriptions
of other lesser dragons, and Appendix 2 lists every lesser
dragon that has been described in a DUNGEONS & DRAGONS rulebook
or accessory.
The become more powerful as they grow older is part of the paragraph that talks only about the MM10. So it just restates that the MM true dragons become more powerful as they grow older.
The next paragraph talks about other true dragons than the MM10, which shows that the previous paragraph was only about the MM10.
So, all true dragons do not necessarily become more powerful as they grow older.
An interesting point, which reduces the definition of a True Dragon to just being a Dragon with 12 age categories. Guess Dragonwrought Kobolds are even less easily excluded. Half-Dragon Kobolds are still excluded because of the specific note in Races of the Dragon.
So, in all of D&D 3.5, Dragon Magic is actually the only place where True Dragonhood is
actually defined. Lesser Dragons were defined in Draconomicon, but not True. Ironic, all things considered, since the book is primarily about True Dragons.
So, recap time.
[spoiler]
For the most part, this book concerns itself with the ten
varieties of true dragon described in the Monster Manual—
the five chromatic dragons (black, blue, green, red, white) and
the five metallic dragons (brass, bronze, copper, gold, silver).
True dragons are those creatures that become more powerful
as they grow older.
Here we have Draconomicon's definition of True Dragons...or rather, it's definition of the MM10 (assuming Laughing Man's analysis would be correct). Even if it does refer to ALL True Dragons, all we have is "become more powerful as they grow older". No mention of Advancement at all. Interestingly, the definition doesn't even require the Dragon type, although it is implied by context.
Then we have Dragon Magic stating that True Dragons are Dragons with 12 age categories. Since in this case, the context doesn't give any preference to one type of True Dragon or another, and all True Dragons do fit this, it is safe to assume that it is a defining rule. It does not conflict with Draconomicon, so Primary-Secondary does not come into play. If it does, then we ignore that True Dragons must have 12 age categories (and indeed, must have the Dragon type!), which leads to far more creatures being True Dragons.
Dragons of Krynn has a similar, though less specific, definition as Dragon Magic. It says nothing that Dragon Magic does not.
Lesser Dragons, on the other hand, are defined as thus in Draconomicon (and are not defined in any other source):
Other creatures of the dragon type that do not advance
through age categories are referred to as lesser dragons (which
should not be taken to mean that they are necessarily less
formidable than true dragons).
Lesser Dragon PCs
Using another creature of the dragon type as a player character
is rather less complicated than using a true dragon.
Such a creature has a set level adjustment and no built-in
progression due to age, so after the character begins play
there is no reason to advance the character as a monster
again. For example, a wyvern character, with a level adjustment
of +4 and 7 Hit Dice, has an ECL of 11 and joins a
party of 11th-level characters to adventure. The wyvern
continues advancing as a character, just like the other characters
in the party.
First off, "other creatures of the Dragon Type" already tells us that to be a Lesser Dragon, you cannot be a True Dragon. To be a Lesser Dragon, you cannot advance through age categories and you cannot have a built-in progression due to age.
Note, however, that nowhere that True Dragons are defined are Lesser Dragons mentioned, nor are "advancing through age categories" or "built-in progressions due to age." So, by what is actually written in the books, a particular creature's status on any of these aspects is irrelevant for determining if a creature is a True Dragon or not. A True Dragon may "advance through age categories" or it may not, but a Lesser Dragon CANNOT. A True Dragon may have "a built-in progression due to age" or it may not, but a Lesser Dragon CANNOT. If a creature fits the requirements for a True Dragon, it can be a True Dragon, but it CANNOT be Lesser.[/spoiler]