My daughter is 5 with a late November birthday. She attends a Montessori school and has spent this year in the equivalent of a kindergarten classroom. Now we are switching to public school and have to decide whether to send her to kindergarten or first grade. Everyone who has met her, including her current teacher, comments on how mature she is, so I don't worry about that. Academically, I'm not as certain - Montessori is different than traditional school so although she does fine there, I worry that she might struggle in first grade...she can read basic words but is by no means reading well. She can count and do basic addition (e.g., 5+3=8) and can write notes to us and has a robust vocabulary. She's above-average in height, is incredibly well-mannered and is appropriately social. Her best friend is also entering first grade (is currently in KG at the school they'll attend) as a legitimate 6 year old. I was also young for my class (September bday) and did fine, but I guess I wonder whether she might become more of a leader or excel if we waited another year for first grade, and I know it's tough to be the last kid to get your driver's license or be playing sports with kids in the class behind you (in city leagues that go by age, not grade). Also, she has two younger brothers and this would make her likely 4 years apart from her brother (he has a Sept bday and I assume we'll wait to send him) who is less than 3 years younger. Any advice?
Based on what you have written, it sounds like your daughter is ready to move on. First grade is when students really learn how to read. So if she has a head start she will probably pick it up naturally. If her present teacher is saying she's ready, then your daughter must have proved that she has mastered the Kindergarten curriculum.
Your own experience will be helpful when she runs into the same problems like getting your driver's license latter than your friends. You will be more understanding and be able to share your own experiences and lessons learned with her.
Retention is a difficult decision, but by talking to your daughter's teacher, other parents, and your family, I'm sure you will make the right decision.