I haven't been posting in a bit (or on this board in some time), but I'll chime in briefly because the situation you describe is very close to my dd15's situation. Her bd made our fall K cut off by two weeks and it is very, very common where I live to hold kids whose bds are in the summer and fall out an extra year. She was the youngest in her grade by a good bit before skipping as a result. I'd say that, on average, 1/4 of her class was 10-18 months older than she before she skipped.
She skipped the last year of elementary (5th) grade and is now 12-30 months younger than her grade peers. She is also now an 11th grader so we're getting close to the end of this K-12 journey. It was absolutely the right decision for her. A good majority of the people who she considers close friends graduated high school last year and are now college freshmen (18-19 years old and 3-4 years older than she).
Socially it worked; academically it worked better than the alternative. The only time I can recall her being in a spot where her age was an issue was last summer. She was 14 and out with her friends who had just graduated high school. They were all 18 and they were going out to a movie. The movie was R rated although something I was fine with her seeing. The person who was taking the tickets was in one of her classes at school and knew that she wasn't 18 and refused to let her in. Her friends all exchanged tickets too, though, and they all went to see something like World War Z (definitely not their first choice) so she wouldn't be left out.
She is pretty centered, though, and not one who is prone to caving to peer pressure or hanging out with kids who would pressure her to drink, for instance. Her friends are all pretty good kids who get together and go to poetry slams or take bike rides or go to swing dancing classes at a yoga studio at night, for instance.
