Angular 2+ Component binded to array inside a ngFor

I've a custom number picker component component with @Input and @Output parameters:

import { Component, OnInit, Input, Output, EventEmitter } from '@angular/core';

@Component({
  selector: 'app-number-picker',
  templateUrl: './number-picker.component.html',
  styleUrls: ['./number-picker.component.css']
})
export class NumberPickerComponent implements OnInit {
  @Input() value: number = 0;
  @Input() min: number = 0;
  @Input() max: number = 9999;
  @Output() output = new EventEmitter<number>();

  enabled1: boolean = true;
  enabled2: boolean = true;

  constructor() { }

  ngOnInit() {
  }

  add() {
    if (this.value < this.max) {
      this.value++;
      this.output.emit(this.value);
    }
    this.enabled1 = true;
    if (this.value == this.max) {
      this.enabled2 = false;
    }
    else {
      this.enabled2 = true;
    }
  }

  remove() {
    if (this.value > this.min) {
      this.value--;
      this.output.emit(this.value);
    }
    this.enabled2 = true;
    if (this.value == this.min) {
      this.enabled1 = false;
    }
    else {
      this.enabled1 = true;
    }
  }

}

It's working well in a simple template.

The problem apperars when its binded to an array inside a *ngFor:

@Component({
  selector: 'my-app',
  template: `
    <div>
      <h2>Picker 1 & 2 seems connected in a strange way</h2>
      <div *ngFor="let age of ages;let i = index">
        <app-number-picker [value]="age" (output)="onAgeChanged($event, i)"></app-number-picker>
      </div>

      <h2>Picker 2 works well if it's outside ngFor but still binded to the array</h2>
      <app-number-picker [value]="ages[1]" (output)="onAgeChanged($event, 1)"></app-number-picker>
    </div>
  `,
})
export class App {
  ages: number[] = [2,2];
  constructor() {
    this.name = `Plunker! v${VERSION.full}`;
  }

  onAgeChanged(value, index) {
    console.log('index', index);
    this.ages[index] = value;
  }
}

You could see than the first two pickers are not working well, but the third (same as second) it's OK.

How can I bind the picker inside the ngFor?

I've a plnkr with the all the code: https://plnkr.co/edit/aNh23TSnkhdfLnm8Ypq1?p=preview

Thank's in advance.