PHP - 44

var_dump(max(count_chars($n))>1?false:true);

Or Proper function 53 Characters

function(){return max(count_chars($n))>1?false:true;}

PHP - 44 33

var_dump(max(count_chars($n))<2);

Or Proper function 42 Characters

function(){return max(count_chars($n))<2;}

UPDATE : Thanks for @scrblnrd3 pointed out for shorter solution.