<script>
$('[name="DOB"]').flatpickr({
    enableTime: false,
    dateFormat: "d/m/Y",
    minDate: new Date().fp_incr(-43800), // 120 years ago
    maxDate: new Date().fp_incr(-6570), // 18 years ago
});

<input name="DOB" type="text"/>

setTimeout(function(){
$('[name="DOB"]').flatpickr({
    enableTime: false,
    dateFormat: "d/m/Y",
    minDate: new Date().fp_incr(-43800), // 120 years ago
    maxDate: new Date().fp_incr(-6570), // 18 years ago
});
},500);