var years = [1990, 1965, 1937, 2005, 1998];

function arrayCalc(arr, fn) {
    var arrRes = [];
    for (var i = 0; i < arr.length; i++) {
        arrRes.push(fn(arr[i]));
    }
    return arrRes;
}

function calculateAge(el) {
    return 2020 - el;
}

function isFullAge(el) {
    return el >= 18;
}

var ages = arrayCalc(years, calculateAge);
var fullAges = arrayCalc(ages, isFullAge);

console.log(ages);
console.log(fullAges);