After fifteen minutes, or if someone posts after yours, then you can no longer edit your post.
If you wish to remove an older post just PM (private message) a moderator or Admin, I am and Admin, and ask to have it removed. You can PM just by clicking a moderators name. All moderators have the designation in their signature.
John is correct on why posts cannot be edited or removed after a period of time. Some people just want to cause problems and post things to cause trouble and then go back and change their posting to make it look like they were innocent.