I signed up when I was 16, as long as you're mature in your posts and you don't try to start stuff you should be okay. But yeah, MK generally is a M rated game so..
I honestly don't think anyone would care for your age if they knew or not. It's not like my generation and below are new to how the world works. Kids do stuff only adults should be doing imo but so much has changed. Just act mature and everything will be fine.