When creating an account with MusicBrainz you need to pick a unique username and choose a password, the username you pick is the only name associated with your account and will be what other MusicBrainz users know you as. You may optionally also tell us the URL of your homepage, and/or a few words about yourself. Any information thus provided is made available to other logged-in MusicBrainz users.

Viewing my own profile when I'm not logged in shows

• User type
• Age
• Gender
• Country
• Member since
• Homepage
• Subscribers
• Bio
• Languages

I'm not sure making a difference between logged-in users and the rest of the world makes that much sense - you only need to solve one captcha to get an account you can use to collect the information you wish. It makes more sense to mention that the provided information is (hopefully) not exported in the data dumps.