*Furiously plays with Google Calculator*
(beat)
AHA!
I have it.
Looking at the Growth curve in the upper right, after about 250 years or so Growth seems to level off into a very shallow angle, but it still becomes asymptotic instead of exponential. I'm going to assume for my own sanity that it doesn't become asymptotic to the x-axis but instead for a very low sloped oblique since Pick's pencil marks aren't exactly clear and seem to indicate that Azzarn shrink past a certain point if you take the arrow point and not the line literally...
The difference between 3000 and 1000 is 8/3 times as much as the difference between 1000 and 250. If we assume this ratio carries on, with the same height changes and the years being X+8/3X where X is the previous age each time, then we end up with:
8,333 + 1/3 is, by my eyeball measures, just under Hoquer's height. If we round up to account for that, 8,400 seems like a nice number.
It also sounds very good. 8,400 years old for Hoquer and Umber really does make them as ancient as mountains. (Adrian Laguna, who is listening to me babble out this theory, demands that I say this is figurative and that Mountains are really much older).
So... Pick... am I close?

Edit- Older? If the curve really does become asymptotic to the X-Axis instead of the one I chose based on the age/height ratios and the growth curve's significant room for interpretation, the ratio of 8/3rds would actually be constantly shrinking. It could potentially not even yield 8'6" height before infinity, although I haven't bothered with the series to figure it out since I'm out of school, not back in Algebra II.
Perhaps 9,000 or even 10,000 to account for that?