Agree. I find DD1 has a great time with kids just a little bit older than her. She seems to let them take the lead more and learns from them (she's used to bossing her sister around, LOL). She loves kids of any age though and it would be a shame to limit that.
One of the reasons I love the Montessori my kids go to is because kids aren't grouped just with kids their same age. DS is in Cycle 1 now and it includes kids from 3-6 years of age. It means that over 3 years he gets to experience being the youngest, the middle and the eldest in the group. At the moment I think it greatly benefits him to have kids older than him. He's quite a solitary child by nature so watching the more refined social skills of the older kids is great for him.