As I have seen, there is no age limit for opening a Steemit account. At least there is no method of verification of the age.
On the other hand, writing on Steemit is not a employment, even if you receive money for it. The product (your writing) is not being sold by Steemit, thus you do not "work" for Steemit. At best, it could be said you are a freelance author selling your writing to Steemit. Also, Steemit does not demand a certain amount of content from a user. You can only read other posts, or comment them.
In view of that, Steemit cannot be regarded as the "employer" of the people who write here, who is responsible for work conditions, under aged workers and similar things.