One thing I noticed after reading that synopsis is that, for purposes of the TV show, they've advanced the ages of all the children by two to three years each. Therefore, it could be that we're supposed to think the rebellion was more like 17 or 18 years ago, rather than 15.
Why did they do this? Some of it is probably a nod to modern sensibilities which would be a little freaked over 11 and 12 year olds getting married. Historically, the book is probably a more accurate version of medieval matchmaking, but there you go.
As a more practical matter, it was probably just a lot easier to work with slightly older actors and actresses - especially since the series could theoretically go on for a few years.