You really are the best judge of what will work for your child. Let me tell you what we did with ours:
At 9.5 years old, our DD started coming home alone after school. She would immediately call me at work on the cell phone. It has worked out very,very well.
We told the school principal and her 4th grade teacher that she was doing this, (so that DD didn't lie to them - although she was not supposed to tell anyone else.)
We had a set of rules that were not to be broken. Period. Or it would have been back to the sitter's house. No 2nd chances, period.
DD is able to get into our hosue with no keys. We also have a neighbor's house she goes to if something happened to the keyless entry things, also if anyone is in our driveway, (i.e. UPS, fedex etc). DD also knows that if the neighbor, for any reason says she is to go to their house, then that is what she is to do - no questions asked- but then call me.
DD loved the independence. She always tried to get her homework done before we came home, which was about an hour later.