CultureInfoとJapaneseCalendarを利用すればよいらしい。

Window1.xaml

<controls:ChildWindow x:Class="SilverlightApplication3.Window1"
           xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
           xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
           xmlns:controls="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls"
           Width="377" Height="184" 
           Title="ChildWindow">
    <Grid x:Name="LayoutRoot" Margin="2,2,2,1">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0" Margin="2">
            <TextBlock Text="元号" Margin="3"></TextBlock>
            <Border BorderBrush="Black"
                    BorderThickness="5"
                    CornerRadius="8"
                    Margin="2"
                    >
                <StackPanel Orientation="Horizontal">
                    <RadioButton GroupName="group1"
                                 Content="M"
                                 ToolTipService.ToolTip="明治"
                                 Checked="RadioButton_Checked"
                                 >
                    </RadioButton>
                    <RadioButton GroupName="group1"
                                 Content="T"
                                 ToolTipService.ToolTip="大正"
                                 Checked="RadioButton_Checked"
                                 >
                    </RadioButton>
                    <RadioButton GroupName="group1"
                                 Content="S"
                                 ToolTipService.ToolTip="昭和"
                                 IsChecked="true"
                                 Checked="RadioButton_Checked"
                                 >
                    </RadioButton>
                    <RadioButton GroupName="group1"
                                 Content="H"
                                 ToolTipService.ToolTip="平成"
                                 Checked="RadioButton_Checked"
                                 >
                    </RadioButton>
                </StackPanel>
            </Border>
        </StackPanel>
        <StackPanel Orientation="Horizontal"
            Grid.Row="1"
            Margin="2">
            <TextBlock
                Margin="2"
                Text="年">
            </TextBlock>
            <TextBox
                x:Name="year"
                Grid.Row="0"
                Grid.Column="1"
                Margin="2"
                Text=""
                MaxLength="2"
                >
            </TextBox>
            <TextBlock
                Grid.Row="1"
                Grid.Column="0"
                Margin="2"
                Text="月">
            </TextBlock>
            <TextBox
                x:Name="month"
                Grid.Row="1"
                Grid.Column="1"
                Margin="2"
                Text=""
                MaxLength="2"
                >
            </TextBox>

            <TextBlock
                Grid.Row="2"
                Grid.Column="0"
                Margin="2"
                Text="日">
            </TextBlock>
            <TextBox
                x:Name="day"
                Grid.Row="2"
                Grid.Column="1"
                Margin="2"
                Text=""
                MaxLength="2"
                >
            </TextBox>
        </StackPanel>

        <Button x:Name="CancelButton" Content="キャンセル" Click="CancelButton_Click" Width="75" Height="23" HorizontalAlignment="Right" Margin="0,12,0,0" Grid.Row="2" />
        <Button x:Name="OKButton" Content="OK" Click="OKButton_Click" Width="75" Height="23" HorizontalAlignment="Right" Margin="0,12,79,0" Grid.Row="2" />
    </Grid>
</controls:ChildWindow>

Window1.xaml.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;


namespace SilverlightApplication3
{
    public partial class Window1 : ChildWindow
    {
        private readonly Dictionary<string, int> m_yearStrToMaxYearValue = new Dictionary<string, int>()
        {
            {"M", 45 },
            {"T", 15 },
            {"S", 64 },
            {"H", 27 },
        };


        public Window1()
        {
            InitializeComponent();
        }

        private string m_yearStr;

        private bool IsValid()
        {
            int max = m_yearStrToMaxYearValue[m_yearStr];
            if (int.Parse(this.year.Text) > max)
            {
                MessageBox.Show("入力エラー");
                return false;
            }


            return true;
        }

        private void OKButton_Click(object sender, RoutedEventArgs e)
        {
            CultureInfo culture = new CultureInfo("ja-JP")
            {
                DateTimeFormat = { Calendar = new JapaneseCalendar() }
            };

            if (!IsValid())
            {
                this.DialogResult = false;
                return;
            }

            string target = String.Format("{0}{1}/{2}/{3}",
                m_yearStr,
                this.year.Text,
                this.month.Text,
                this.day.Text
                );

            var d = DateTime.Parse(target, culture);

            this.Dispatcher.BeginInvoke(() =>
                {
                    MessageBox.Show(d.ToString("yyyy/MM/dd"));
                });
            this.DialogResult = true;
        }

        private void CancelButton_Click(object sender, RoutedEventArgs e)
        {
            this.DialogResult = false;
        }

        private void RadioButton_Checked(object sender, RoutedEventArgs e)
        {
            var cur = sender as RadioButton;
            m_yearStr = cur.Content.ToString();
           
        }
    }
}