I went for an interview with a very large company, I had filled in my application online and was asked to review it. It already asked if I was over 18. The girl interviewing asked me to fill in my birthdate and SS# by hand. After I was finished with the interview she said a manager would call me for a 2nd- has not happened yet! ANYWAY I was under the impression that until you are HIRED your birthdate and SS# are not needed. My feeling is the manager saw my age and thought I am too old, Can an employer ask for this info BEFORE you are hired?

No, I've been on a million interviews in my lifetime, and you always need to fill out everything on your application, including the particular information you are questioning. So, in the future, be sure to do this.
To some employers, by not filling in this information, they may think you are trying to hide information, etc...
By her telling you that a manager will be calling you for a 2nd interview-doesn't necessarily mean that someone is definitely going to call you, it just means that should they decide to call you in for a 2nd interview, a manager, someone other than her, will be calling to notify you.
Good luck with everything!

Thanks for the info, I was confused about what they can and can not ask. THANKS!

This site has some specifics

http://www.allstarresume.com/careercenter/illegal_interview_questions.asp

Also, if you Google interview questions you will find a wealth of tips.

Asking a person's age in the application or interview is illegal. That application is outdated and they should not be using it. You can only be asked that information after you start working.

I am in the HR field and no, they cannot ask your date of birth, year you graduated, etc. As far as your social, employers who run preemployment background checks will need that to do so.