I placed my daughter in the year ahead as the school recommended her for it. She graduated from HS at age 16 and went to Jr. College. I thought she was too young for a regular dorm at a university.
Scholastically and socially she did very well. I also was a year ahead and always the youngest in my classes. For me it was tough socially.
I would not have allowed my youngest daughter to be a year ahead. Developmentally she was about three years behind herself and only caught up the last year of HS.