یک if بزارید اگر مقدار بالای 20 بود ثبت نکنید 

میتونید سمت کلاینت هم کنترل کنید ، با استفاده از Jquery

میشه نمونه کدش رو لطف بکنید

مثال

function getAge(dateString) {
  var now = new Date();
  var today = new Date(now.getYear(),now.getMonth(),now.getDate());

  var yearNow = now.getYear();
  var monthNow = now.getMonth();
  var dateNow = now.getDate();
 //date must be mm/dd/yyyy
  var dob = new Date(dateString.substring(6,10),
                     dateString.substring(0,2)-1,                   
                     dateString.substring(3,5)                  
                     );

  var yearDob = dob.getFullYear();
  var monthDob = dob.getMonth();
  var dateDob = dob.getDate();
  var age = {};
  var ageString = "";
  var yearString = "";
  var monthString = "";
  var dayString = "";


  yearAge = yearNow - yearDob;

  if (monthNow >= monthDob)
    var monthAge = monthNow - monthDob;
  else {
    yearAge--;
    var monthAge = 12 + monthNow -monthDob;
  }

  if (dateNow >= dateDob)
    var dateAge = dateNow - dateDob;
  else {
    monthAge--;
    var dateAge = 31 + dateNow - dateDob;

    if (monthAge < 0) {
      monthAge = 11;
      yearAge--;
    }
  }

  age = {
      years: yearAge,
      months: monthAge,
      days: dateAge
      };

  if ( age.years > 1 ) yearString = " years";
  else yearString = " year";
  if ( age.months> 1 ) monthString = " months";
  else monthString = " month";
  if ( age.days > 1 ) dayString = " days";
  else dayString = " day";


  if ( (age.years > 0) && (age.months > 0) && (age.days > 0) )
    ageString = age.years + yearString + ", " + age.months + monthString + ", and " + age.days + dayString + " old.";
  else if ( (age.years == 0) && (age.months == 0) && (age.days > 0) )
    ageString = "Only " + age.days + dayString + " old!";
  else if ( (age.years > 0) && (age.months == 0) && (age.days == 0) )
    ageString = age.years + yearString + " old. Happy Birthday!!";
  else if ( (age.years > 0) && (age.months > 0) && (age.days == 0) )
    ageString = age.years + yearString + " and " + age.months + monthString + " old.";
  else if ( (age.years == 0) && (age.months > 0) && (age.days > 0) )
    ageString = age.months + monthString + " and " + age.days + dayString + " old.";
  else if ( (age.years > 0) && (age.months == 0) && (age.days > 0) )
    ageString = age.years + yearString + " and " + age.days + dayString + " old.";
  else if ( (age.years == 0) && (age.months > 0) && (age.days == 0) )
    ageString = age.months + monthString + " old.";
  else ageString = "Oops! Could not calculate age!";

  return ageString;
}

// A bit of jQuery to call the getAge() function and update the page...
$(document).ready(function() {
  $("#submitDate").click(function(e) {
    e.preventDefault();

    $("#age").html(getAge($("input#date").val()));

  });
});