javascript, age, keydown

I already have onkeydown for one function called dateformat

My goal is if someone enter date format like 01/01/1972, it will call another function called 'CheckAge' and I want to show the value inside of the <div id="getage"></div>

How can i do that?

 function DateFormat(txt, keyCode) {
        if (keyCode == 16)
            isShift = true;
        //Validate that its Numeric
        if (((keyCode >= 48 && keyCode <= 57) || keyCode == 8 ||
             keyCode <= 37 || keyCode <= 39 ||
             (keyCode >= 96 && keyCode <= 105)) && isShift == false) {
            if ((txt.value.length == 2 || txt.value.length == 5) && keyCode != 8) {
                txt.value += seperator;
            }
            return true;
        }
        else {
            return false;
        }
    }

 function CheckAge(dateString)
    {
        if (dateString != '') {
            var today = new Date();
            var birthDate = new Date(dateString);
            var age = today.getFullYear() - birthDate.getFullYear();
            var m = today.getMonth() - birthDate.getMonth();
            if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
                age--;
            }
            alert(age);
        }
        else
        {
            alert("No Age");
        }
        return age;
        
    }

<div id=GetAge></div>
 <div class="ui-input">
											<input type="date" required tabindex="3" maxlength="10" name="dob" id="dob" onkeydown="return DateFormat(this, event.keyCode)" placeholder="Enter DOB" value="<%= GetDOB(Request.QueryString["Primary DOB"]) %>" class="form-control" />
											<label class="ui-icon"><i class="fa fa-birthday-cake"></i></label>
                                             <p id="exactAge"></p>
 										</div>

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