DD11 started K early at 4 1/2 and she was 3 months younger than our cut off. Since this was suggested by her pre-k teacher, we just went along with it without further thought. When that wasn't enough though, I began to feel like a pest at the school, since they thought early entrance should be enough.
Many parents think they are helping you by volunteering negative information. My favorite answer has been "we are taking it year at a time". Most of the time this will end the comments and I quickly change the subject. It must be hard for other parents to understand what it is like to raise a gifted child. I would love to just send her to school and her age would predict the correct grade for her. She is now taking classes at 6th, 7th and 8th grade hopping all over the building as we try to find the right classes.
I think you should go with your gut as you know your child the best.
Jen