When can they tell the sex?

Megan • 24, have a 6 year old girl and an amazing boyfriend! Expecting #2 November of 2015!!
I keep seeing people say around 12-18 weeks they found out what the sex of their baby is, but I thought it was more like 20 weeks, and I sure I was 20weeks with my daughter...When did you ladies find out? I'm only 7 so I have a ways to go regaurdless.