I want this to apply to the age input box and to only except the values that are between 1 - 100,in 'javascript'.
<h1>Form</h1>
<form name="form" onsubmit="myFunction()" method="post" action="">
<div>
<input type="text" required minlength="3" maxlength="20" placeholder="Enter your first name">
</div>
<div>
<input type="text" minlength="3" maxlength="20" required placeholder="Enter your last name">
</div>
<div>
<input id="age" required name="number" type="number" placeholder="Enter your age">
</div>
<div>
<input type="text" required placeholder="Enter your email">
</div>
<div>
<input type="submit" required value="Submit">
</div>
</form>
The attributes min and max is probably what you're looking for.
<input id="age" required name="number" type="number" placeholder="Enter your age" min="1" max="100">
You may need to clarify what you mean by "only except the values that are between 1 - 100,in 'javascript'." if not.