WPF MVVM TextBox绑定后台字符串

本人WPF菜鸟，我想把TextBox控件数据绑定在Person类的Age字段上，但界面并不能更新，请大神帮忙，谢谢

界面绑定代码：

界面类部分代码：
MainViewModel mainViewModel = new MainViewModel();
public MainWindow()
{

        InitializeComponent();
        grid.DataContext = mainViewModel;

    }

ViewModel部分代码：
public MainViewModel()
{
this.ShowInfoClick = new DelegateCommand(ShowInfoExec);
this.UpdateInfoClick = new DelegateCommand(UpdateExec);
this.Name = p1.Name;
this.Age = p1.Age;
this.Hobby = p1.Hobby;
}

    public void UpdateExec()
    {
        p1.Age = p1.Age + 1;
        this.Age = p1.Age;
        this.Name = p1.Name;

    }

    private int _age = 24;
    public int Age
    {
        set
        {
            _age = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("Age"));//对Age进行监听  
            }
        }
        get
        {
            return _age;
        }
    }

Person类部分代码：
public class Person:INotifyPropertyChanged

{
private String _name = "张三";
private int _age = 24;
private String _hobby = "篮球";

    public int Age
    {
        set
        {
            _age = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("Age"));//对Age进行监听  
            }
        }
        get
        {
            return _age;
        }
    }

MainViewModel 是否有继承INotifyProperyChanged接口？