Sir Ian McKellen is 85 years old, and one can see why the renowned British actor took on the lead role in the new movie “The Critic.” The part affords him the chance to play a range of emotions, deliver the occasional tart line of dialogue, and generally commandeer every scene he’s in — the kind of role not many over-80s are offered. Unlike, though, this summer’s hit “Thelma” with the 94-year-old June Squibb in a sparkling role, Mr. McKellen’s film leans on tragedy instead of comedy, thereby turning what could have been wickedly articulate fun into a mishmash.