Mine are homeschooled, so no experience there, but I was "youngest kid" and don't regret it. My only suggestion (based on my own school experience, not my kids') is to make this decision based exclusively on her academic level (or average academic level) rather than concern for her being the same age as the other kids, which I don't think matters that much. If she tends towards the behind end of school skills, keep her back; if she tends to be ahead or average, I would put her in.