One wild theory suggests that franchise's historical artifacts.
The first film, Raiders of the Lost Ark, saw Indiana Jones recover and witness firsthand the power of the Ark of the Covenant. The second film, Indiana Jones and the Temple of Doom, served as a prequel, which saw Indy encounter the powerful Sankara stones and the murderous Thugee cult. The fourth film introduced sci-fi elements by having Jones deliver an alien's crystal skull to its ship in the ruins of an ancient city. However, perhaps most importantly, Indiana Jones and the Last Crusade involved the search for the Holy Grail itself.
One theory explained (via Indiana Jones 5, but also how he survived one infamously unbelievable moment from Kingdom of the Crystal Skull. Though there is evidence that the Grail may not have made Indiana Jones truly immortal, it does seem that it may have given him some form of restorative powers.
The Last Crusade does specify that the Grail only offers immortality to those who do not leave its temple. Jones does just that in order to save the life of his father, losing the Grail in the process. However, as Jones Sr. dies offscreen before Kingdom of the Crystal Skull, it's clear that immortality is likely out of the question. However, one of Kingdom of the Crystal Skull's worst mistakes - the infamously ridiculous scene in which Indiana Jones survives a nuclear blast by hiding in a refrigerator - does imply that the hero may be protected by some divine force.
As The Last Crusade establishes the Grail's restorative powers, it's not unreasonable to assume that it has continued effects on Jones. It's possible that it doesn't stop him from aging but does give him the ability to recover from otherwise lethal situations, like a nuclear blast. As the Grail brought Jones Sr. back from the brink of death, this sort of power very much makes sense within Indiana Jones' wider narrative.
Not only would the theory clear up that infamous Kingdom of the Crystal Skull plot hole, but it would also make an interesting narrative development in Indiana Jones 5. The idea that Indiana Jones' adventuring has had a lingering effect on him would help introduce the idea of the character's reported final outing through subtext but also allow for an interesting callback to previous movies. The fact that the Grail almost certainly didn't offer genuine immortality is also important, as Jones will still eventually die, but the theory goes a long way to explaining how Indiana Jones was able to inexplicably survive so many close calls after The Last Crusade.
