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

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/

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>