I never saw Underdark being a low level adventure. In any computer game or P&P module.
"Out of the Abyss" - is an offitial WoTC module that starts in Underdark at lvl 1.
You are stuck in a mindset of leveled zones. Which is not how DnD areas work. They provide environments and possible problems. Which parts of it player's encounter is DM's decision.
