I was 19 weeks. Usually at your anatomy scan they find out ;)
some as early as 15 but not as accurate. avg 16-18 weeks
They start telling the sex at about 16 weeks.
Usually its at ur anatomy ultrasound which is 18-20 wks. Some ppl get the blood test to find out the sex that way too which can b done sooner. Not sure if insurance covers it tho.