В этой части кода есть событие TextChanged для включения кнопки на панели приложений.

C #:

private void Textbox_TextChanged(object sender, EventArgs e)
{
    ApplicationBarIconButton btn_guardar = ApplicationBar.Buttons[0] as applicationBarIconButton;

    if (!string.IsNullOrEmpty(txt_nom_usuario.Text) && !string.IsNullOrEmpty(txt_edad_usuario.Text) && !string.IsNullOrEmpty(txt_peso_usuario.Text))
    {
        btn_guardar.IsEnabled = true;
    }
    else
        btn_guardar.IsEnabled = false; 
}

XAML:

<phone:PhoneApplicationPage.ApplicationBar>
    <shell:ApplicationBar Mode="Default" IsVisible="True">
        <shell:ApplicationBarIconButton x:Name="btn_guardar" IconUri="/icons/appbar.save.rest.png" Text="Guardar" Click="btn_guardar_Click" IsEnabled="False" />
        <shell:ApplicationBarIconButton x:Name="btn_atras" IconUri="/icons/appbar.back.rest.png" Text="Atrás" Click="btn_atras_Click" />
    </shell:ApplicationBar>
</phone:PhoneApplicationPage.ApplicationBar>

<TextBlock x:Name="lbl_ingresanombre" Height="39" Margin="60,28,0,0" TextWrapping="Wrap" HorizontalAlignment="Left" VerticalAlignment="Top" Width="248" FontSize="29.333" FontFamily="{StaticResource Helvetica}"><Run Text="Ingresa "/><Run Text="tu nombre"/></TextBlock>
<TextBox x:Name="txt_nom_usuario" Height="63" Margin="47,58,69,0" TextWrapping="Wrap" Text="&#xa;" FontSize="21.333" VerticalAlignment="Top" IsEnabled="True" />

<TextBlock x:Name="lbl_edad" Height="38" Margin="60,117,0,0" TextWrapping="Wrap" Text="Ingresa tu edad" VerticalAlignment="Top" FontSize="29.333" HorizontalAlignment="Left" FontFamily="{StaticResource Helvetica}"/>
<TextBox x:Name="txt_edad_usuario" InputScope="TelephoneLocalNumber" Height="63" TextWrapping="Wrap" Text="&#xa;" FontSize="21.333" Margin="47,147,69,0" VerticalAlignment="Top" MaxLength="3" />

<TextBlock x:Name="lbl_peso" Height="42" Margin="60,0,0,178" TextWrapping="Wrap" Text="Peso" HorizontalAlignment="Left" VerticalAlignment="Bottom" Width="74" FontSize="29.333" d:LayoutOverrides="HorizontalAlignment" FontFamily="{StaticResource Helvetica}"/>
<TextBox x:Name="txt_peso_usuario" InputScope="TelephoneLocalNumber" Margin="47,0,69,125" TextWrapping="Wrap" Text="&#xa;" FontSize="21.333" Height="63" VerticalAlignment="Bottom"/>