Dd2 is an aug 15 bday. We did not put in K this last fall at 5. Registration for K is in March. Last march dd had just started talking to kids at preschool less than a month before - she's painfully shy. Academically she would have been ok but not great. She had zero interest in her letters (didn't know more than a handful) last spring. She got interested over the summer and we could have gotten her academically ready but barely but socially she would have been a mess.
I know we made the best decision for her. At preschool she went from never participating or talking to now she is one of the more popular girls (even though she still doesn't like all the girls flocking around her but she'd never dream of hurting anyone's feelings). She has hit all the ideal academic milestones for K with confidence already. She will probably be just enough ahead by fall so it will be easier for her to adapt to a new (bigger) school and new kids for awhile and more or less ignore the academics. She struggles to adapt to new places so that is something she'll need time to do without worrying about getting behind with academics.
I know by waiting a year she'll do awesome at school instead of just ok.