A way to generate random birthdays in MongoDB shell.

randomBirthdays.js

db = db.getSiblingDB("e1_learn-qa");
//db.getCollection("profiles").find({});
var oldest = 24 * 60 * 60 * 1000 * 365 * 18;
var newest = 24 * 60 * 60 * 1000 * 365 * 5;
var randomDate = function () {
    return new Date(Date.now() - (Math.random() * (oldest - newest)))
}

db.profiles.update(
    {dob: {$exists: false}},
    {$set: {"dob": randomDate()}},
    false,
    true
)