"Fred and George tried to get me to make [an Unbreakable Vow] when I was about five. I nearly did too, I was holding hands with Fred and everything when Dad found us. He went mental, only time I've ever seen Dad as angry as Mum. Fred reckons his left buttock has never been the same since."
Since Ron was five, Fred and George would only have been seven years old. How would they have been capable of (nearly) getting Ron to make an Unbreakable Vow? Their parents would certainly not have told them about Unbreakable Vows, and the Weasleys were not the sort of wizards to own a book on the subject, let alone to leave it lying around where the twins could get hold of it. Possibly they could have learned from one of their older brothers, but I doubt that either one would have been older than thirteen or so--not old enough to know much advanced magic (if an Unbreakable Vow is actually advanced magic). In any case, Unbreakable Vows were not taught at Hogwarts, much less how to make them. How would the twins have figured it out?