Our daughter is 12 and is in the fifth grade. She is much more mature than her classmates and is thus, having difficulty making friends.
Looking older or developing earlier than classmates can be difficult, and unfortunately is out of her control. But you can help her focus on her interests and talents. Encourage involvement in those areas of interest.
Her association with others who have similar interests will automatically create a peer group. Based on her interests, you can get her involved in athletics or perhaps art courses or music classes. Involvement in her areas of interest can be both in school and out of school.