Tell them when you want!
With my first we told everyone straight away, we were too excited to hold it in!
Think we were the same with my second, this one we did plan to keep it quiet till I was 12 weeks, but we went away with family at 8 weeks and thought we better tell them incase something happened while we were away, we also wanted to tell the kids and knew they wouldn't be able to keep it quiet for the next 4 weeks!

Snoopea