My introvert is more uncomfortable with older kids than my extravert. Her preference in situations when she is not with a friend, is for adults or children who are much younger and are open to being "mothered". She is very shy about approaching same age peers or older kids.
I've actually always wondered about the "prefers the company of older kids" that is mentioned in a lot of gifted checklists. I wonder how much age and gender impact that generality. DD really liked and played well with girls who were older than her when she is toddler/preschool age. As she got older it seemed to me that even if I put the shy factor aside, she would have preferred younger to older if she was playing, because younger kids were more interested in imaginative, creative play while older girls were interested in things she found stupid and boring (e.g pop culture, make up, etc.). I recall a similar issue for the first gifted girl I worked with as a teacher. She was hard pressed to find another 4th/5th grade girl who still wanted to engage in imaginative play (they were more likely to walk around the playground giggling about who had a crush on who).
It seems like the social play of boys is less age sensitive, at least in elementary school, so DS (who is pretty tall anyway) has a blast running around with boys of any age.
Thinking about the OP's situation, I also wonder about the difference between structured and unstructured associations. Several people have mentioned individual vs. group dynamics.
I realize that any gender comments are overgeneralizations. I'm just wondering about trends. Anyone else have any thoughts re: age/gender?