I wonder if for an extroverted kid, who likes to lead play and be in the thick of things, older kids just aren't as satisfying? They're possibly already getting to play their own choice of games, they're able to be take control of the kinds of interactions they are having etc.
Mine is an extrovert, so maybe this has something to it.
The funny thing is that DD can also be a "pet" of older kids in some situations. I think she likes them a lot individually but doesn't like to be completely lost in a sea of them, as some of the stuff others talked about (older kids grouping up) happens then.
