From a historical perspective, they're at the perfect age to get married as many of our ancestors were probably between 11-14 when they married too, and surely not all of those marriages ended in chaos and disaster because they were too young when they got married. I don't see anything *inherently* wrong in this, necessarily. Lots of people far older than 11 make incredibly stupid decisions to marry incredibly wrong people for incredibly wrong reasons, so preteens definitely don't have a monopoly on stupid marriage decisions.
That said, I don't see the point. People live a lot longer these days, childbirth is a lot easier, and you don't need to have a lot of kids to help on the farm or to ensure the continuation of the family line. Those being the big 3 reasons for marrying so young, there isn't a lot of reason left over. Also working against them are modern labor laws, which didn't exist back then, so it's not like either of them will be able to provide each other a life together until much later anyway.
So while I don't necessarily think it's too young from an objective standpoint (I wouldn't personally do it or like it if my hypothetical imaginary children did it, but whatev, two Saudi kids' marriage doesn't affect me and I hope they can pull it off), I do think it's kinda silly and unnecessary and probably more than a little driven by the adults in charge of them. I wonder why they don't just get betrothed or something and save the actual marriage 'til later.
I'm also really curious about the full story there, though, 'cause the article's pretty brief and vague.