According to Rashi, Our Father Yitzchak married a three-year old. Rashi's view isn't the only view - the Ibn Ezra, for instance, argues that Rivka was much older when Eliezer appeared at her well- but it's the view all of our kids are taught in school. Every year, at Parshas Chayei Sarah, the small children come home repeating this obscene story, and only the most intelligent among them think to point out that the idea of a 40 year old man hooking up with a tiny-tot is more than a little gross. The rest presume it's normal and natural. In fact, some of the more romantically-minded children may wonder why no forty-year old man has yet come courting for them.
I'm not equipted to decide between Rashi and the Ibn Ezra. By my lights, both make strong arguments. The text supports them both. I'm merely pointing out that if you're a kindergarten teacher who wishes to teach your students one fact about Yitzchak, the fact that Rashi thinks he was married to a three-year-old might not be the one you should choose. But if you're a teacher who, for your own bizarre reasons, feels it's absolutely crucial to tell your class how old Rivka and Yitzchak were on their wedding day, why not follow the Ibn Ezra?