BuddyPress.org

I am busy deciding on whether to incorporate terms of use in my site.

1) Why do so many websites say you have to be at least 13 ?

Would it be a problem to allow any age?

2) Can I simply copy another websites terms of use and modify it a bit?

3) Do your users have to agree to terms at signup or can you simply put terms in a tab on your homepage and then assume people agree to it?

Thanks.

Just want some general input.