Are Indy and Henry immortal?
No. The easiest way to think of the Holy Grail is not as the cup of immortality, but as the fountain of youth. One sip won't make you immortal; you have to keep drinking from it. That would explain why the knights who left the cave after drinking from the Grail eventually died of extreme old age. Nothing in the film states that one becomes immortal by drinking from the Grail one time, and if that were the case, it would be difficult to explain those knights' deaths. The grail knight does say that the grail itself cannot pass beyond the great seal, for that is the price of immortality. The meaning is that, if you wish to be immortal, you have to stay in that cave for eternity with the grail. Therefore, they are not immortal. Additionally, we see in Indiana Jones and the Kingdom of the Crystal Skull that Henry Jones, Sr. has indeed passed away in the time since the events of Last Crusade.