User can manage one page accessible by everyone?

As you may or may not be aware WordPress provides a profile page in the admin for each registered user (including the admin of course!): `http://www.example.com/wp-admin/profile.php‘

On that page you will see a number of fields that can be edited / customized including a Biographical Info field.

To fetch and output a field from this page for a particular user you can use the get_user_meta function like so:

$bio = get_user_meta( $user_id, $key );

$key could be the string description which corresponds to the Biographical Info field.

You may want to take a look at the get_user_meta Codex page (might be of special interest to output available data so you can correlate fields from the admin with valid $key values based on fields content).