Regarding: " The reason I'm asking is because I may post ads according to age, gender, location, ect. I'm looking for other demographics that may help in posting certain kinds of ads as they visit the sites I have."
What niche are you in? What is the purpose of your website? Why do you need people to register? Is your website safe for people to provide you with their "personal" information?
I agree with chrishirst: "Know your audience"
Btw, if you run a website just to collect others' information for your advertisement purposes, spammers and scammers sites like that I don't waste my time visiting!
Where do your website stand? Is it a site people can get something out of it on every visit, or is it just a website for you to steal others' information?