public class RegisterUserViewModel

    {

        public UserInfo userInfo { get; set; }

        public ICommand ClickCommand { get; set; }

        public RegisterUserViewModel()

        {

            userInfo = new UserInfo();

            userInfo.Age = ;

            this.ClickCommand = new DelegateCommand<object>(OnClick, arg => true);

        }

        void OnClick(object obj)

        {

            userInfo.Age += ;

        }

    }

<Window x:Class="mvvmTest.MainWindow"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        Title="MainWindow" Height="" Width="">

    <Grid>

            <Button Height="" HorizontalAlignment="Left" Margin="12,0,0,72" Name="btnClick" VerticalAlignment="Bottom" Width="" Command="{Binding ClickCommand}">Click me</Button>

            <TextBlock Height="" Margin="12,44,0,0" Name="textBlock1" VerticalAlignment="Top" Text="UserName:" HorizontalAlignment="Left" Width="" />

            <TextBlock Margin="12,93,0,0" Name="textBlock2" Height="" HorizontalAlignment="Left" VerticalAlignment="Top" Width="" Text="Age:" />

            <TextBlock Height="" Margin="99,44,117,0" Name="tbUserName" Text="" VerticalAlignment="Top" />

        <TextBlock Height="" Margin="99,93,117,0" Name="tbAge" Text="{Binding userInfo.Age}" VerticalAlignment="Top" />

    </Grid>

</Window>

    public partial class MainWindow : Window

    {

        private RegisterUserViewModel viewModel;

        public MainWindow()

        {

            InitializeComponent();

            this.ViewModel = new RegisterUserViewModel();

        }

        public RegisterUserViewModel ViewModel

        {

            get

            {

                return this.viewModel;

            }

            set

            {

                this.viewModel = value;

                this.DataContext = value;

            }

        }

    }

    public class NotificationObject : INotifyPropertyChanged

    {

        public event PropertyChangedEventHandler PropertyChanged;

        public void RaisePropertyChanged(string propertyName)

        {

            if (this.PropertyChanged != null)

            {

                this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));

            }

        }

    }

    public class UserInfo : NotificationObject

    {

        private int age;

        public int Age

        {

            get

            {

                return age;

            }

            set

            {

                age = value;

                this.RaisePropertyChanged("Age");

            }

        }

    }