Did you include "gains HD through aging" or "grows in size through aging"?
Reposting:
True Dragons:
1. Advance through age categories (Drac, pg 4)
2. “Become more powerful as they grow older” (Drac, pg 4)
3. Resembles a reptile at first glance. It has a muscular body, a long, thick neck, a horned or frilled head with a toothy mouth, and a sinuous tail. The creature walks on four powerful legs with clawed feet, and it flies using its vast, batlike wings. Heavy scales cover a dragon from the tip of its tail to end of its snout. (Drac, pg 5)
4. Every true dragon, no matter how large or small, has exactly 13 cervical vertebrae, 12 thoracic vertebrae, 7 lumbar vertebrae, and 36 caudal vertebrae. (Drac, pg 8)
5. All true dragons are endothermic. (Drac, pg 9)
6. The shortest-lived true dragon, the white, can live as long as 2,100 years. The true dragon species that lives the longest is the gold; Guillaume and Cirjon put the gold’s maximum age at 4,400 years. (Drac, pg 15)
7. Can swim (Drac, pg 21)
8. Every true dragon is immune to at least one type of elemental energy (acid, cold, electricity, or fire) (Drac, pg 22)
9. True dragons have superb internal temperature regulation and seldom suffer from the effects of excessive heat or cold. (Drac, pg 22*)
10. True dragons also develop a supernatural resistance to physical blows, which can prevent nonmagical weapons from harming them at all. (Drac, pg 22*)
11. Thanks to their innately magical nature, true dragons also develop the power to shrug off the effects of spells. Older dragons ignore spell assaults from all but the most powerful magical practitioners. (Drac, pg 22*)
12. *Also on pg 22 is a sidebar that clarifies all true dragons do not have to make Fort saves every 10 minutes as long as the temperature is between -40 and 140 Farenheit, develop damage reduction as they age, and develop spell resistance as they age.
13. All true dragons have great patience. (Drac, pg 25)
14. True dragons fall into two broad categories: chromatic and metallic. (Drac, pg 36)
15. All true dragons have the potential to use arcane magic. (DoE, pg 30)
16. True dragons are winged, reptilelike creatures of ancient lineage. They are known and feared for their size, physical prowess, and magical abilities. (MM1, pg 68)
17. The known varieties of true dragons (as opposed to other creatures that have the dragon type) fall into two broad categories: chromatic and metallic. (MM1, pg 68)
18. All true dragons gain more abilities and greater power as they age. (Other creatures that have the dragon type do not.) (MM1, pg 68)
19. True dragons gain spellcasting power as they age… (RotD, 111)
So to answer your question, I did include advancement through age categories (point 1), but I must have missed the "range in size" bit. So add
20. Range in length from several feet upon hatching to more than 100 feet after attaining the status of great wyrm. The size of a particular dragon varies according to age and variety. (MM1, pg 68)
Unfortunately, since several types of True Dragon (eg White, Steel, etc) don't meet this point , it is useless when determining the definition of True Dragon.