Javascript Date Validation ( DD/MM/YYYY) & Age Checking

I'd utilize the built in Date object to do the validation for me. Even after you switch from - to / you still need to check whether the month is between 0 and 12, the date is between 0 and 31 and the year between 1900 and 2013 for example.

function validateDOB(){

    var dob = document.forms["ProcessInfo"]["txtDOB"].value;
    var data = dob.split("/");
    // using ISO 8601 Date String
    if (isNaN(Date.parse(data[2] + "-" + data[1] + "-" + data[0]))) {
        return false;
    }

    return true;
}

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/parse#Example:_Using_parse for more information.

If you want to use forward slashes in the format, the you need to escape with back slashes in the regex:

var pattern =/^([0-9]{2})\/([0-9]{2})\/([0-9]{4})$/;

http://jsfiddle.net/P9TER/

If you want to use forward slashes in the format, the you need to escape with back slashes in the regex:

   
var dateformat = /^(0?[1-9]|1[012])[\/\-](0?[1-9]|[12][0-9]|3[01])[\/\-]\d{4}$/;

I suggest using moment.js which provides an easy to use method for doing this.

interactive demo

function validate(date){
    var eighteenYearsAgo = moment().subtract(18, "years");
    var birthday = moment(date);

    if (!birthday.isValid()) {
        return "invalid date";    
    }
    else if (eighteenYearsAgo.isAfter(birthday)) {
        return "okay, you're good";    
    }
    else {
        return "sorry, no";    
    }
}

To include moment in your page, you can use CDNJS:

<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.4.0/moment.min.js"></script>