This is a question where the same answer will not work for every person involved because we all mature at different rates, according to body chemistry, DNA, and other environmental factors, like where you live or if you are gay or straight. There are so many different combinations of sexuality, and therefore it is hard to give any real input on when everyone should start dating. You just have to look at the person really.
I first started dating in grade 4, because some boy asked me out. I said no to him, he pouted, and then I said yes because we were close friends at the time. We never acted like a couple though. After that day it was never brought up again, and then I saw him asking some other girl out on the playground. I think her name was Kelly. Either way, we never spoke about it and that is how it went. Obviously I was too young to even have come into my sexuality at that point, because I am a lesbian.