[BNB]

Решение через функцию:

//** mdate: [object Date] **//
  
  function checkAge(mdate) {
    var age =  mdate.getFullYear()-1995;
    var prefixes = {
      "0": "лет",
      "1": "год",
      "2": "года",
      "3": "года",
      "4": "года",
      "5": "лет",
      "6": "лет",
      "7": "лет",
      "8": "лет",
      "9": "лет"
    };
    
    if (age >= 11 && age <= 14) {
      document.write(age + " " + "лет");
      
      return;
    }
    
    var strAge = age + "";
    var lastNum = strAge.slice(strAge.length-1);
    
    document.write(age + " " + prefixes[lastNum]);
  }