What age category would you say a dragon would have to be to use it's skull to make an appropriately sized helmet for your average medium sized creature?
I'd say two size categories larger should be enough. The head isn't the largest part of the dragon, so a Large size dragon wouldn't have a big enough skull to help.
While the head of a dragon may not be abnormally large, neither is it abnormally small. If the skull of a lion is large enough to make a helmet than making one from a dragon’s skull should not be a problem. I think that a huge skull would actually be too large to make a medium sized helmet. An elephant is a huge animal and its skull would be way too big for a medium helmet.
A dragon is not the same as a lion or elephant. At least based on common depictions, dragons usually have much larger bodies in comparison to their heads than those animals, and their wings and tails make up some of their mass as well.
I definitely think the head of a Large dragon would be smaller than that of a lion, and be too small for a human to wear easily.
Considering that a dragon’s bite damage does more damage than a similarly sized creature, I don’t think dragons have small heads. Looking at the illustration on the cover of the core rule book also does not look like the dragon has a small head. It actually looks larger than normal.
The 3.5 Draconomicon and one of the issues of Dragon magazine address what a dragon's corpse can be used for based on its size, include the bones for weapons and armor.
I'm away from my books, so I can consult it, but maybe someone else could.