Dragon familiar - HD, age & size?

[jaw6]

I'm working on a level 10 "fire wizard"-type character, and I'm curious about the dragon familiar (from Draconomicon) works. When it says:

A dragon familiar that becomes very young in age can no longer serve as a familiar.

how does a dragon familiar age? Is it based on HD or something else?

I was hoping to have a brass dragon familiar (to go with the fire theme), but it's not clear to me how it would work.

 

[HeavenShallBurn]

Notice that as a familiar none of the abilities being granted the wyrmling are actual HD, they're all virtual by way of reflection from the caster's HD/BAB/etc.

As such I'd say the familiar ages normally by racial HD, in which case you get 5 years until it's no longer a wyrmling.

 

[jaw6]

Ah, okay, I wasn't used to thinking with "virtual hit dice", but that sort of makes sense. Thanks!