The brothers walked out of the desert 150 years later or whatever, I think it's safe to assume they'd all stayed behind ... probably building the temple, setting up the booby traps, etc. Rome wasn't built in a day, ya know.
So Indy (and his Dad, for that matter) had one swig from the cup. That probably revitalized him and kept him from aging a day. That's all. Had they stayed behind the seal yet never drank from the cup again, they'd probably have no extension to their life span. Like most fountains of youth in literature, immortality is a double-edged sword -- granted only to those who slavishly drink from it for all eternity, addicted to it, knowing that abandoning it will mean certain death.
Just my interpretation.