I had the same problem but this workaround worked for me. (I have just bought an iPhone 5s for my 11-year old, but my wife and I both have Androids, so had no access to an alternative IOS device.)
I was able to initially sign into my daughter's new iPhone using my own AppleId from years ago, then was able to set up my account for family sharing, then signed out again and signed back in using my daughter's fresh account.
More details: once I'd signed in as me, I went to Settings -> My Name -> Family Sharing and went through the steps to create a new account for my daughter. (I had to do a software update for IOS first, as the option in Settings wasn't there until after the update had completed.) Then I signed out, then signed back in using my daughter's new account, and that seemed to work.
I expect the same thing would work if you don't already have an existing adult account, presumably you can just create one for you initially, then follow the same steps.
(I know this is an old post, but it came up in my search for an answer, so it's still worth me replying as others will likely come across this article in future.)