You can file anytime after you turn 18... You don't necessarily have to file before you turn 18 for N-600.
I just sent in my N-600 and I'm 33, but my mom naturalized when I was 15 (about 18 years ago, wow time flies). I had no idea derivative citizenship existed so I just finally got my act together to get the CoC.
Also, via this board, the forums here, I found out an even easier way to prove your citizenship is to apply for a US Passport. As long as you have your mother's naturalization certificate, your birth certificate, and proof that you were/are an LPR (green card) at the time she was naturalized, you can apply for a passport, which has a 2-4 week turnaround, vs. the N-600 which has a 5-6 month turnaround. Look for another thread I started for more info (I'd post the URL here but the forums won't let me post a URL yet b/c I'm a new registrant).
Hope that helps and good luck lazyboy.