I like the extension User Extended Field and with some help of you guys I got it working quite well.

Now I have another question:
How can I make it possible to show some specific field only on the public profile page, when it's the users own profile. For example: When I login I want to see my own birthday, but others can't see my birthday when the check out my profile.

In default.php from the UserExtendField plugin I already tried this:
The 'JRequest::getInt('id') is right, but $user->id gets me the wrong id from the k2_users table. In this table there is a column 'id' and a column 'userID' and I want to grab the 'userID'.

I hope there is someone who can help me with this!

You can show/hide this information by using Joomla!'s user function.

docs.joomla.org/Accessing_the_current_user_object

In this post you will see the how this function is used to get a specific user.