jaelis
Oh this is where the title goes?
Is the Ancient GD really a different kind of creature than the young one? Is the Archmage the same kind of creature as the Human Commoner? The thing is you seem to allow the TP into a quite specific type of Gold Dragon but not a specific type of human. How?
What kind of perspective are you thinking about?
Most narrowly, I would let you turn into an ancient dragon because I would let you turn into an ancient human. The polymorph spell doesn't address age, but that's how I would play it.
A little more broadly, I think that polymorph in general is not supposed to give you class abilities. There are plenty of good reasons for that in terms of game balance and playability. I interpret that broadly, in the sense that I think most of the special NPC abilities represent class abilities, even if they aren't called out specifically. But I wouldn't interpret general monster features that way.
From a game logic perspective, I would explain disallowing class features because they are products of experience, not biology. I think polymorph changes your body and gives you the basic skills needed to use that body, but it doesn't provide the lifetime of study and practice that a human needs to become an archmage. In contrast, I assume that an ancient dragon's abilities are more a product of biology, in that any dragon who lives long enough will gain them. If you felt that some of those abilities were instead learned, then you might not want to grant them. At the same time then, I would expect to find dragons in your world with varying ability sets.