5. The INSANELY Clumsy Dialogue
Would it be an M. Night Shyamalan movie without some embarrassingly on-the-nose, stilted dialogue?
Old's biggest overall weakness is surely its screenplay, which survives more on the strength of its deranged premise than the quality of its moment-to-moment dialogue.
From the very first scene, characters make howlingly unsubtle allusions to ageing and death, nods which Shyamalan presumably thinks are slyly massaged into scenes organically, but this couldn't be further from the truth.
Characters also exposit at great length about both their own melodramatic backstories and theories on why they're all rapidly ageing, and very little of it actually sounds anything like something a real human being would say.
Again, Shyamalan's lucky that the central premise remains so thoroughly gripping regardless.