Did you watch Prometheus and ask yourself, 'Hey, why bother putting Guy Pearce in crummy old age make-up instead of just casting an old guy? Well, Guy Pearce wants to help you understand. While doing promotion for his new movie Lawless, the Aussie actor told reporters that there was supposed to be a scene in which David enters the slumbering Peter Weyland's dream and meets his maker in the pose of a young man. '[W]e didn't end up shooting it. We got close to shooting it, and as we got closer, Ridley just started to see it as something that would just be a little distracting, almost.'
So there you have it. Possibly the worst explanation, but an explanation nonetheless. If you wanted to know what Pearce probably would've looked like as young Weyland, check out the viral video of his character's Ted talk, circa 2023, above.