JSFiddle - React, Tailwind, and code Playground

HTML

Summed ages: <span></span>

JavaScript

"use strict";

let data = JSON.parse('[{"employee":[{"firstname":"Harry","birthdate":"1965-10-05","age":50}]},{"employee":[{"firstname":"Pete","birthdate":"1985-10-09","age":30}]}]');

document.querySelector('span').innerText = sumAllAges(data);

function sumAllAges(data) {
    return data.reduce(function(total, obj) {
        return total + obj.employee[0].age;
    }, 0);
}