The minimum age for moderator without needing an exception is 15. You can be any age to get an exception, provided Chad feels you're mature enough to handle it.
Here are a few tips from different people and mods I've talked to. As well as different guides I've read.
Application:
Welshy's guide to becoming a moderator: Go for quality over quantity. Even if you write a ten thousand word essay on why you should be a mod, if it's not good quality, it will be denied.
PKTran: They tend to look for somewhere between 900-1200 words, but any amount is fine if it's good quality.
Duckluv's guide: Be patient. It may take awhile for them to look at your application.
KellieBreanne: Hang around with mods, you'll pick up on certain things that can help, such as the length of bans, how to recognize illegal mods or hacks, and how moderators should act in general.
Everyone: Don't lie on your application.
Interview:
Welshy's guide: This is when you go into extreme detail when answering questions. One sentence answers can really hurt you.
BioGear94: Use TeamSpeak's push to talk feature. That way you won't talk over they people asking questions.
McMorpheus: Take a moment to think of your answer before you reply to a question. That way you won't ramble.
Mr.Dice: Don't read things straight off your application when asked questions. If possible go into more detail than what your application says.
Everyone: Don't be nervous.
General:
KellieBreanne: Act like a mod even before you post your application. You know, helping people out with problems, sounding intelligent, and just being an all around nice person.
Duckluv: Try to be active on both the forums and the servers.
I hope this helped.