JSFiddle - React, Tailwind, and code Playground

JavaScript

function validateAge(birthday, minage, separator) {
    if (!parseInt(minage)) {
        return false;
    }
    var split = birthday.split(separator);
    var dateFullYear = split[2];
    var dateMonth = split[1];
    var dateDay = split[0];
    var mydate = new Date();
    
    mydate.setFullYear(dateFullYear, dateMonth-1, dateDay);
    
    var currdate = new Date();
    var setDate = new Date();         
    setDate.setFullYear(mydate.getFullYear() + minage, dateMonth-1, dateDay);

    return (currdate - setDate) > 0;

}

var valid = validateAge('09/09/2018', 18, '/');
console.log(valid);