What level do your farms need to be at before you can start attacking them?

You can attack at any level but you will not be generating a lot of rss to collect until they reach around 12 maybe. You will have to decide when they have something useful to take. Be careful not to hit them too hard when they are weak as they will end up teleporting away automatically if you zero them.

Let your farm grow until it's base 16. Upgrade your storage and then start hitting it. Its a longer process but you get better yield this way.