[Vuejs]-How to validate a single field inside a form element-ui?

For individual field validation,

<script src="//unpkg.com/vue/dist/vue.js"></script>
<script src="//unpkg.com/element-ui@2.13.2/lib/index.js"></script>
<div id="app">
<el-form :model="ruleForm" status-icon :rules="rules" ref="ruleForm" label-width="120px" class="demo-ruleForm">
  <el-form-item label="Age" prop="age">
    <el-input v-model.number="ruleForm.age"></el-input>
  </el-form-item>
  <el-form-item>
    <el-button type="primary" @click="submitForm('ruleForm')">Submit</el-button>
    <el-button @click="resetForm('ruleForm')">Reset</el-button>
  </el-form-item>
</el-form>
</div>

<script>
var Main = {
    data() {
      var checkAge = (rule, value, callback) => {
        if (!value) {
          return callback(new Error('Please input the age'));
        }
        setTimeout(() => {
          if (!Number.isInteger(value)) {
            callback(new Error('Please input digits'));
          } else {
            if (value < 18) {
              callback(new Error('Age must be greater than 18'));
            } else {
              callback();
            }
          }
        }, 1000);
      };
      return {
        ruleForm: {
          age: ''
        },
        rules: {
          age: [
            { validator: checkAge, trigger: 'blur' }
          ]
        }
      };
    },
    methods: {
      submitForm(formName) {
        this.$refs[formName].validate((valid) => {
          if (valid) {
            alert('submit!');
          } else {
            console.log('error submit!!');
            return false;
          }
        });
      },
      resetForm(formName) {
        this.$refs[formName].resetFields();
      }
    }
  }
var Ctor = Vue.extend(Main)
new Ctor().$mount('#app')
</script>