As an extensive user of Codecademy, I have been thoroughly satisfied with the service they provide. It is in their terms of service that the site is for individuals 13yrs and up, which is not something that they enforce, and from my experience interacting with the staff, I don't believe they really feel that it is an issue. But, to be a completely legal user, you should be at least thirteen years old.
Not only do they have an excellent method of teaching, they also have moderators for the Q&A Forums that do a wonderful job of responding to users that are having issues, as well as keeping the Forums clear of spam and unwholesome content. I very highly recommend the site, as a way of learning to code second only to personal training.