Moderator is a promotion after Helper. To become helper, apply here:
Apply
You are required to be
14+ years old and have been on the server for
1 month minimum.
Mod would come after a while as helper with a good record on forums and in game (for me, it was roughly 5 months).
In regards to what @iiPandaMan said, Build Team does not have an age limit, but rather looks at your builds, and you will most likely undergo a build test from Yogos, the Build Team Leader, if your application is considered (only the best builders are accepted, so its very competitive).