Let's take a look at Med'an, he's half Draenei, usually, with half breeds lifespans lay in the middle between the two parenting races. Med'an couldn't be older than 23 or so? So perhaps somewhere around the age of 30 Draenei become mature? Seems odd because they live so long, but the Draenei race is always on the run, perhaps because of that they grow to maturity so quick?
Med'an isn't half draenei. He's quarter draenei.
His mother is half draenei, half orc, while his father is presumed to be Medivh.
Genetics also do not work in a 'halving' way. =P There's no reason to assume that a quarter draenei would live to be as old as a quarter of a Draenei's age, and mature four times quicker.
The vast majority of Draenei are, in fact, older than the oldest Night Elves. Most of these Draenei don't look young at all! There is a great reason to believe that Draenei, while perhaps not immortal, live for tens of thousands of years. This being the case, a Draenei in their thousands would be quite young. I think they make mention that one of the child NPCs is hundreds of years old somewhere (Children's day maybe?).