Trying PHP IF Else Statements for the first time

I'm just learning PHP and I'm trying to understand the if else statements a little better, so I'm creating a little quiz. However, I have come across an issue and I don't seem to know what the issue is. My problem is that whenever I type in the age in the input area, it will give me the $yes variable every time even if I enter the wrong age

Here is my code so far:

my .html file

<form action="questions.php" method="post">
<p>How old is Kenny?<input></input>
<input type="submit" name="age" value="Submit"/>
</p></form>

my php file

<?php
$age = 25;
$yes = "Awesome! Congrats!";
$no = "haha try again";

if ($age == 25){
    echo "$yes";
}else{
 echo "$no";
}
?>

Thank you in advance for your time.