My doctor office does the u/s to find out the sex at the 18-20 week mark. Some places do them as early as 16 weeks but I think to be sure they try to wait until 18-20. I am 18 weeks right now and my appt for that is nov. 8th so I will find out about 20 weeks.

I found out I was having a boy when I was 19 weeks and 3 days

Remember, it's a guess at any stage. The later the u/s, the more accurate the guess. But always allow room for error.