问题描述：我有下面这样一个数组， 数组里面每个对象有三个相同的 key ：name、age、hair。

let namelist = [{
  name: 'mark',
  age: 15,
  hair: 'long'
}, {
  name: 'tuwen',
  age: 16,
  hair: 'short'
}, {
  name: 'xiaoming',
  age: 16,
  hair: 'short'
}, {
  name: 'lilei',
  age: 15,
  hair: 'short'
}, {
  name: 'hanmei',
  age: 17,
  hair: 'long'
}]

筛选条件：数组中age相同的对象分到一个数组里。

期望结果：

arr_1 = [{
  name: 'mark',
  age: 15,
  hair: 'long'
}, {
  name: 'lilei',
  age: 15,
  hair: 'short'
}]

arr_2 = [{
  name: 'tuwen',
  age: 16,
  hair: 'short'
}, {
  name: 'xiaoming',
  age: 16,
  hair: 'short'
}]

arr_3 = [{
  name: 'hanmei',
  age: 17,
  hair: 'long'
}]