[Blackheart]

Jeff mentioned one of the those old books and it reminded me of one I dont think anyone has mentioned. In the main character is a space hero of some sort and in part of the stoy encounters 2 scenarios that cause him to reverse age and perhaps die of old age. The first ending comes from when he fights a scorpion-like monster. Its sting cause him to rapidly become younger until, if I recall, he ceases to exist. His memory and perspective also shifted with him.
The other endings involved (of course) a alien princess and a evil overlord. The overlord kept dominion because of a simple looking ray gun. This gun not only gave him immorality, but could regress or age anyone that displeased him. The hero fails in 2 ways during this part. The first being he thinks the gun is a joke and gets blasted, i believe, causing him to die of old age. The second ending causes a fight between the two leaving the overlord a pile of dust, but leaving the hero an infant in the care of the princess (she'll watch him until he can grow up to save the universe).
Personally, I loved these old books, and while I appreciated Goosebumps take on them, I prefer the classics. I may one day hunt them all down!