There are, for all intents and purposes, three types of questions on the registration form:
Each type of question has different rules relative to your ability to hide the questions.
You can't hide standard questions. Standard questions are required on the registration form.
That said, if you really want us to remove the Date of Birth question from your registration form, contact us and we can do it for you. All other fields have to stay.
You can opt to hide Emergency Contact Info, Medical Info and Estimated Finish Time by unchecking the respective box on the Custom Questions page of Event Setup. (Don't know how to access the custom question page? Learn here)
If you have custom questions already created and you no longer want to show them in registration, all you need to do is click the "hide" button next to the specific question and - POOF! - it will disappear from the registration form. (Don't know how to access the custom question page? Learn here)