There's no rule about this. It depends on what effect you're going for. If your story functions in a large world with many characters, or you want the main character to stay distant and mysterious at first, you can put off having him or her on the page for a while. But that will make readers think someone else is the main character, and it could be jarring when/if the story seems to suddenly shift. If that's the effect you're going for, then okay. Just be aware it could turn some people off.
That said, a lot of books open with someone other than the main character. Crime books tend to do this a lot; I rarely like it when they do. Some fantasy or historical epics that have a gazillion characters do this. I tolerate it more here because I know we're being immersed in a larger world and have more patience to wait and see who the main characters are.
You might want to resist the urge to flash back or tell a lot of background in the first chapters of the story unless your plot depends a lot on memory, or the book isn't chronological. Weave background details into scenes that have their own forward drive, with goals for the characters on the page. There should always be a balance of show and tell, and how you balance it depends on your genre and style, but in general, drip feeding as little background as necessary is usually the best way to go. In the end, all that background you thought was so important early in the story often isn't.