I know what you mean. I went to a birthday party with my smallish DS5.10, and the differences in physical size and maturity with his first-grade classmates were obvious. You are wondering about challenges with asynchronous development that many parents here have faced, so I would just keep up with the forum, search for helpful info, and ask questions as they come up.
I also think, in a general sense, that it's a good idea to hold on instead of letting a raptor literally soar away with one's child in its talons. But that's the sort of tension between environmentalism and parenting that some here may have faced as well.
The difficulty in sending one's kid off to kindergarten is one faced by a great many parents too, of course. I think there's nothing to it. Tears are often shed, you pack the kid off, and in the end the experience is most likely going to turn out all right. Here are some pics of our doleful DS5 going off to his first day of kindergarten-- but that night he was thrilled to have met so many new people:
http://photos.smugmug.com/IMG0045/994591762_yt29m-O.jpghttp://photos.smugmug.com/IMG0088/990263568_n7rsZ-O.jpg