You can use Javascript to do that:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Welcome to My Page!</title>
</head>
<body>
<div id="about-me"></div>
</body>
<script>
function get_message() {
var my_birthday = new Date("2/13/1996"); // update your birthday here
var today = new Date();
var time_diff = Math.abs(today.getTime() - my_birthday.getTime()); // get the difference in milliseconds between today's date and your birthday
var my_age = Math.floor(time_diff / (1000 * 3600 * 24 * 365)); // get your age
var s = document.getElementById('about-me');
var message = "I’m a " + my_age.toString() + "-year-old part-time blogger & designer and full-time waitress & bartender!";
s.innerHTML = message; // update your age
}
window.onload = get_message; // load script only when page is loaded
</script>
</html>
Hope this helps!