Yeah, I only recently got the option. Been waiting on it for ages. Thankfully it coincided with the start of the CATER series.
Crackn81 also got the option, about a week before I did. So I don't know what the criteria is. He has a lot more subs, and a lot more video views than I do, so it makes no sense to me. As far as I understand it now, so long as you are monetising at least one video, they consider you a partner. And you get things unlocked over time. I still don't have the custom button above the video option (like extreme601 has).