JSFiddle - React, Tailwind, and code Playground

HTML

<label for="id_nested-1-birthdate">Birthdate</label>
<input class="dateinput" datadatepicker="datepicker" id="id_nested-1-birthdate" name="nested-1-birthdate" type="text" />
<div id="alert" class="alert alert-warning"></div>

JavaScript

Ago50Y = new Date();
Ago50Y.setFullYear(Ago50Y.getUTCFullYear()-50);

$('input.dateinput').change(function (event) {
    if ($(event.target).val() < Ago50Y.toJSON().slice(0,10)) {
        $('#alert').text('This person is over 50 year. Remember to do...');
    } else {
        $('#alert').text('');
    }
});