<script type="text/javascript">
var mdate = new Date();
var years = ["лет","год","года","года","года","лет","лет","лет","лет","лет","лет","лет","лет","лет","лет","лет","лет","лет","лет","лет","лет","год","года","года",]; // и т.д.
var age =  mdate.getFullYear()-1995;
document.write(age + " " + years[age]);
</script>

//** 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]);
  }

<script>
var plural = function(b) {
    return function(a) {
        return b[1 == a % 10 && 11 != a % 100 ? 0 : 2 <= a % 10 && 4 >= a % 10 && (10 > a % 100 || 20 <= a % 100) ? 1 : 2]
    }
};
var d = plural([' год', ' года', ' лет'])
var mdate = (new Date()).getFullYear() - 1995;
document.write(mdate + d(mdate));
</script>

<script>
f=n => n+" "+['год', 'года', 'лет'][(n%100>4 && n%100<20)?2:[2, 0, 1, 1, 1, 2][(n%10<5)?n%10:5]];  
var age = new Date().getFullYear()-1995;
document.write(f(age));
</script>