Apparently low spy(common form of an attack build) stats makes your end gold lower. As the end result sort've averages out for a reward it means that low spies = low income = drop in average = low gold return.
Not confirming this myself because I've only heard it... Post here and my wall if you know for sure.
You have to be over 18 and send an application. It's recommended you're a mature player who enjoys taking time to help the community and has time to be active. I am unsure if there is a rule regarding how long you've been playing, however it is most likely a factor.