JSFiddle - React, Tailwind, and code Playground

HTML

<form name="as400samplecode">Name:
    <input type="text" name="namee" required>
    <br>
    <br>gender:
    <input type="radio" name="sex" value="male">Male
    <br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    <input type="radio" name="sex" value="female">Female
    <br>
    <br>
    <input type="text" name="myDate" size=10 maxlength=10 oninput="myAgeValidation()">(in MM/DD/YYYY format)
    <br>
    <br>
    <br>
    <br>
    <input type="checkbox" name="agree" disabled>I, Agree
    <br>
    <br>
    <br>
    <input type="button" value="Submit">

CSS

.acceptable-date {
    border : green 1px solid;
}
.valid-date {
    border : pink 1px solid;
}
.invalid-date {
    border : red 1px solid;
}

JavaScript

function myAgeValidation() {

    var lre = /^\s*/;
    var datemsg = "";

    var inputDate = document.as400samplecode.myDate.value;
    inputDate = inputDate.replace(lre, "");
    document.as400samplecode.myDate.value = inputDate;
    datemsg = isValidDate(inputDate);
    if (datemsg != "") {
        //alert(datemsg);
        document.as400samplecode.myDate.className = "invalid-date";
    } else {
        //Now find the Age based on the Birth Date
        document.as400samplecode.myDate.className = "valid-date";
        getAge(new Date(inputDate));
    }

}

function getAge(birth) {

    var today = new Date();
    var nowyear = today.getFullYear();
    var nowmonth = today.getMonth();
    var nowday = today.getDate();

    var birthyear = birth.getFullYear();
    var birthmonth = birth.getMonth();
    var birthday = birth.getDate();

    var age = nowyear - birthyear;
    var age_month = nowmonth - birthmonth;
    var age_day = nowday - birthday;

    if (age_month < 0 || (age_month == 0 && age_day < 0)) {
        age = parseInt(age) - 1;
    }
    //alert(age);

    if ((age <= 25) && (age >= 20)) {
        document.as400samplecode.agree.disabled = false;
        document.as400samplecode.myDate.className = "acceptable-date";

    } else {
        alert("age limit is 20 - 25");
        document.as400samplecode.agree.disabled = false;
    }

}

function isValidDate(dateStr) {


    var msg = "";

    var datePat = /^(\d{1,2})(\/|-)(\d{1,2})\2(\d{4})$/;

    var matchArray = dateStr.match(datePat); // is the format ok?
    if (matchArray == null) {
        msg = "Date is not in a valid format.";
        return msg;
    }

    month = matchArray[1]; // parse date into variables
    day = matchArray[3];
    year = matchArray[4];


    if (month < 1 || month > 12) { // check month range
        msg = "Month must be between 1 and 12.";
        return msg;
    }

    if (day < 1 || day > 31) {
        msg = "Day must be between 1 and 31.";
        return msg;
    }

...