M. Night Shyamalan's new ending was underwhelming (spoilers)
From memory, in the French comic that 'Old' is based on, everyone dies, leaving the baby (now an old man) alone on the beach. The actual cause of the rapid aging is never revealed, the focus of the story is on how people of various ages and walks of life would react if they were suddenly faced with their own mortality as it rushed towards them.
For some reason, M. Night was attracted to this story ... but had to tack on a schlocky mad scientist ending that explains, in explicit detail, what is going on. Don't get me started on the two kids managing to escape the beach (which was under surveillance) and alert the police.
I thought it was a really blunt, crudely constructed way to end the film. It's like M. Night knew he couldn't deliver the downbeat ending of the comic in an artful or satisfying way, and tacked on something he thought a mainstream audience would accept.