Well, let's look at humans and translate that to D'ni.
What's the average age of a human?
So, about 28 years old. That seems young, but let's roll with it. This is hardly scientific, anyhow.
Now, let's say the average lifespan of a human is around 70. This means that the average age is 40% of the lifespan.
Now we take 40% of the D'ni lifespan and we should get an idea of the average D'ni age (assuming D'ni are similar to humans).
It comes out to 146.
Okay, again, this is hardly scientific. We know D'ni women are only able to have a child for a very short period of time. This means that there would likely be more older D'ni than younger. If anyone can give a more scientific answer, that would be great. At least this gives a ballpark number.
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-Whilyam