Click your name at the top bar (or, alternatively, enter your home and click the mirror). You'll see a screen with your stats, a button to change your nickname, a link to change your flag, etc. At the top there's an entry marked "level", with a question mark link. Click the question mark.
You'll see a minimum level of your opponents, depending on your own level. It does start with the three level difference, but it diverges later, so that when you're level 100, you can attack anybody who's level 84 or higher. For a level 41, the minimum opponent level is 33, and you're 34, so you qualify.
