Never go back, they say. But just occasionally, actors do. Charlton Heston cameoed as an aged chimp in Tim Burton’s 2001 remake of Planet of the Apes, while the recent Ghostbusters reboot featured Bill Murray and Dan Aykroyd in minor parts. Harrison Ford has restored his A-list status simply by reprising the roles that made him famous four decades ago.

But if it is relatively rare in Hollywood for the grizzled elder to revisit the hallowed territory where they once reigned supreme as a lead, it is even rarer for an actor to keep playing the same role all the way from youth to dotage. Few sagas last the test of time so successfully as to allow for their stars to age on camera, but that doesn’t mean we won’t see it happening in future – especially in this era of cinematic universes, which in theory can go on and on.

Spider-Man: Homecoming, out in the UK this Friday, offers up a suitable template for such innovations, if Robert Downey Jr can only be persuaded to play ball. The 52-year-old is once again hinting at a departure from the role of Tony Stark/Iron Man, saying that he has one eye on “hanging up my jersey before it’s embarrassing”. But there’s no reason for him to step aside, even if the current swath of Marvel movies keeps running until 2050.

Iron Man himself, metal body armour and all, only appears fleetingly in Homecoming. But the billionaire industrialist, with his haphazard mentoring of the Pater Parker, is crucial to the movie’s success. Stark offers Tom Holland’s teenage wall-crawler something to aspire to – a job with the Avengers – and gives off an avuncular rock-star cool that contrasts nicely with Parker’s deeply green early efforts at superheroism. Iron Man’s relative absence doesn’t really register, but the film would be damaged deeply if Stark had taken the day off.

You have to wonder whether Marvel has considered allowing Downey Jr to take on the mentor role full-time once the next Avengers sequel, Infinity War, is done. By then, the actor will be 54, and perhaps more suited to a role just left of centre stage. In the comics, Stark handed over the metal suit to 15-year-old Riri Williams, AKA Ironheart, last year. It would be hardly be a shock if something similar happened in the on-screen MCU.

One of the reasons Marvel has been the studio’s determination to secure its key cast on long-term contracts. It has meant that Thor has always been played by Chris Hemsworth, Black Widow by Scarlett Johansson, Captain America by Chris Evans and so on. In the rare instance where actors have baulked at a return – Terrence Howard as War Machine, Edward Norton as the Hulk – producers have moved smartly to line up credible replacements. All this adds to the MCU’s sense of internal logic, and it’s hard to argue that this would be further buoyed by allowing Downey Jr to continue growing old disgracefully in these films, even as younger actors take on the more central roles.

There needn’t even be a definitive retirement of that famous suit. Downey Jr could be forced back into action in emergencies, or might even perform his heroism remotely – a trick he pulls off with aplomb in Homecoming. If a new Iron Man really is needed, there’s every argument that Stark should be the man to choose his successor and perform a subsequent mentoring role. In the comics, Tony even became the AI advisor to Williams’ Ironheart after being left in a coma.

The cinematic universe era has the potential to solve many of modern Hollywood’s starkest problems, offering studios the safety of audience familiarity without the need for endless, increasingly insipid sequels and remakes. If it can also remove the need for producers to recast iconic roles every time an actor grows too old, there may be no end to its bounties.