The issue with that feat is that it just improves your ability to wear magic items. If getting the ability to make new items is just improving the DWK's crafting ability the feat to wear more magic items should be an improvement of the normal true dragons ability to wear magic items. And I do not think this it is reductio ad absurdum here just because the case is unusual, under certain conditions true dragons would fail your definition of gaining new abilities so that definition cannot be true by your own universality requirement.
Fair enough, that's not how I read it, but fair enough.
On the spell casting issue I read that not as improving the dragons spell casting level, just their caster level, so they would never gain the ability to cast spells just from aging in the epic advanced progressions, although certain feat trickery could allow that. I would argue that after that a certain point the true dragon would just improve their existing spell casting ability and not gain any new abilities given previous definitions ruling out new spell slots as individual abilities. What if a DWK picks up casting or manifesting trough a feat due to their first hd, lets say hidden talent, aging would give them more times per day to use that power. Or uses the magical training, versatile spell caster, heighten spell, extra slot trickery. Even through they do not gain those HD due to aging they could gain the ability to use spells in those slots due to aging or more spell slots due to aging without the help of any classes spell casting.
While true, there is a limit to how far this can go. Once the DWK reaches a certain point, he no longer gains anything by aging. Also, I think it's very important that aging grant the whole ability, not some combination of aging and gaining class levels.
A UATD, if immortal, can conceivably do this ad infinitum. So, if the DWK were to somehow gain immortality, let's say, by taking that epic feat for expanding your lifespan repeatedly, he still would no longer gain anything after his 3rd ability score boost. While not stated, I think this is the spirit of the statements regarding advancement through age.
I would say to forget about the "new abilty vs. improvement" thing, but the "gains more abilities" thing just nags at my logic. If it had said "gains greater abilities" or some such instead, this would be easier. Then we could just apply the universality question to the DWK, and it would inevitably fail.