let total = 0;
const adults = [];

for (let user of users) {
    if (user.age > 18) {
        adults.push(user);
    }
}

for (let adult of adults) {
    total += adult.age;
}

console.log(total); // => 94

const isAdult = user => user.age > 18;
const prop = key => obj => obj[key];
const add = (a, b) => a + b;

const total = users
    .filter(isAdult)
    .map(prop('age'))
    .reduce(add, 0); // => 94