function sortByAge(arr) {
  arr.sort((a, b) => a.age - b.age);
}

let ivan = { name: "", age: 25 };
let petro = { name: "", age: 30 };
let mariya = { name: "", age: 28 };

let arr = [ petro, ivan, mariya ];

sortByAge(arr);

//  : [ivan, mariya, petro]
alert(arr[0].name); // Ivan
alert(arr[1].name); // Mariya
alert(arr[2].name); // Petro