I created a program in Nxt.js to play age from date of birth, but it doesn't show up.

Cannot read property 'age' of undefined

<template>
approximately
 <td>{{this.age}} years old</td>
approximately
</template>

export default {
  data(){
    US>return{
      age:',
  },
  created(){
    This.$store.dispatch('setStudentsRef', db.collection('students'));
    // Calculate Age //
    const studentRef=db.collection('students');
    studentRef.get().then(querySnapshot=>{
      querySnapshot.forEach(birthDayDocSnapshot=>{
        const birthdayDay=birthDayDocSnapshot.data().pickerDate;
        const today = moment();
        letty=Math.floor(moment.duration(today.diff(birthday))).as('months')/12);
        This.age=y;
      });
    });
  },