We have not told the older children yet either, but have told just about everyone else. Once we tell the kids, EVERYONE will know, including the few people that we have not told.
We are planning to tell them on Friday before our first midwife visit, at our house. We are hoping to hear the heartbeat then and would like them to be a part of it...and they may wonder what the hec is going on if we don't tell them! This will put us at 12 weeks when we tell them.
With a younger child, I really don't think it matters as much. My dd turned 2, 2 months before my ds was born and she didn't really seem to get it anyway. I mean, she knew there was a baby in there and she would point to my belly and say, "baby"...but in general she was pretty oblivious. When he came he just kind of fit into our lives and there wasn't really a problem with adjustment for her.