Ok. This one is a little odd. But, you see, he's not really ten! He just stopped his physical growth at age ten, so that when the time came, he could be with Sakura. So, really, he's much closer mentally and emotionally to Kaho's age, and therefore, this really isn't illegal. ^_^ Actually, they are a really good couple. Anyone could see why he would be in love with Kaho...she's absolutely gorgeous, smart (she's a teacher!), very spiritual, and as an added bonus, has magical powers just like he does. (Ok, well, maybe he's a little more powerful...the point is they can understand each other!) And if you can get over the fact that Eriol looks like he's ten years old, he's got a lot to offer himself. He is quite charming, rich, sophisticated, and, of course, has really powerful magical abilities. Not a bad package, especially when you consider that (eventually) he'll look like Clow Reed.
They met when Kaho left Touya and went overseas. Unfortunately, they had to spend a lot of time apart. She left Eriol to come back and help Sakura with the Clow Cards, according to his plan. She left again before he arrived on the scene. They weren't reunited until the very end of the series.