I'm very close with my boss and all my coworkers. They all knew we were trying...so I told them all within a couple of weeks after I found out. Having the extra support and understanding around the office is really nice.
Me again, that's a good point that smr08 made. It really does depend on the relationship you have with your boss. If you're comfortable with him/her and share a confidence together, then it's really your call.
I would say that it depends on the relationship that you have with your boss. You can do the standard or if you have a good relationship you can tell him/her sooner.
I waited til after my 1st trimester. I scheduled all my appts during lunchtime hours or late in the day. I was considered high risk because of my age as well. Honestly, it doesn't matter what anybody says or thinks, but for your own peace of mind, I would wait til after your 1st trimester. Good luck!
I would wait as long as I could.