Ian McKellen in his early eighties plays the Dane in his mid-twenties. A production with such a strange innovation should be conventional in all other details so that the virtues and demerits of the experiment can be judged in the right context. But Sean Mathias’s show adds extra puzzles. Elsinore is a modern palace ruled by Claudius, in a charcoal suit, and Gertrude in a chic emerald dress, pinched at the waist.