JSFiddle - React, Tailwind, and code Playground

HTML

<div id="d"></div>

JavaScript

var array = [{"name":"Joe", "age":17}, {"name":"Bob", "age":17}, {"name":"Carl", "age": 35}]
var unique = {};
var distinct = [];
    for( var i in array ){
     if( typeof(unique[array[i].age]) == "undefined"){
      distinct.push(array[i].age);
     }
     unique[array[i].age] = 0;
    }
var d = document.getElementById("d");
d.innerHTML = "" + distinct;