Originally Posted by
Biomega
True, but we don't know how old she is as a child, I don't think. For all we know, she could already be 50 or whatever and still appear childlike to us. Nothing says Draenei maturity must be completely linear, it could come in spurts or whatever. Of course, for simplicity it's often assumed most races simply mature the same way humans do. I doubt the devs thought much about it.
That says nothing in and of itself. Most immortal beings depicted in fiction do, in fact, show physical progression of some kind or they'd all be babies if they never ever aged at all; elves in WoW, for example, seem to stop aging around what would be human age 25-30 or so, i.e. peak physical condition. But in the end it's just a matter of how you handle the immortality, and whatever fictional explanation you come up with. The physical changes observed in Velen could also simply be related not to natural aging but to something like emotional state (in humans hair has been known to lose color as a result of stress, for example), or to his exposure to the Light (much like Fel changed the appearance of the Legion-aligned Eredar). Or it's just a story-telling device, to make clear to the player that this is quite some time in the past.
Of course, it's also entirely possible that you're right and they're not immortal, just very long-lived.