I can assure you being young/new doesn't exclude you from decrepitude.
The Underdark is a big place, I think that this is an instance where just because the game only gives you so many ways to get to the Underdark and that particular slice of it, we can assume there are more, either ones we can't find or ones outside the area we're given.
Just to clear up, I can remember these ways to get to the Underdark
Ladder underneath temple of Selune
Zhentarim Elevator
Jumping down from Spider Matriarch's Lair
Portal in Hag's Lair (Only Goes up last time I tried)
I think it's also safe to assume that all the Absoluters we've met have been making it to the surface via the lantern route, so over land to the EA area, like the drow who kidnapped the Duke, Minthara and Nere
...As for why the Zhents are there, aren't they the major suppliers of surface slaves and goods to the underdark now that the trade with Thay has collapsed? (because would you buy food from zombie farmers, it's a bit icky, and probably contaigous)
This is supported by the Halfling Zhent we find in the temple.
