I think being with age peers can work on a lot of levels when they are in K, depending on birthday and such. I think a lot of social learning goes on. But by grade 1, I had enrolled DD in online math to accelerate her. I probably could have done it in K, but she was also very pushed in piano so there was something that was seriously challenging going on and she was a young K.
DD8 is now in 3rd grade and her school is pretty good, she has Spanish and computer lab. But I continue to accelerate her math through CTY (johns hopkins) and she takes Chinese. We decided to slow down the piano. She was expected to compete this year where she could of performed at Carnegie Hall, but she didn't love it and it was serious piano. She still takes piano, but like other kids. She focuses on dance. She has 4 dance classes a week, 2 are preprofessional ballet. I tell her that she has to do something that pushes her and develop good habits.
Between the accelerated online math, 2 languages she has a good program for now. And I am finding that her social learning is pretty intense so I am sort of glad to have her with age mates.
Good luck. Each child here is different. Take what works and leave behind what doesn't. You have to find a path for your child.