My ob/gyn with my first DD made me wait for 22 weeks... excrutiating wait!
Second ob/gyn with DS did it at 20 weeks.
Third ob/gyn with DD2 did one at 16 weeks but wasn't positive about the gender and did it again at 18 weeks.
I've always read that 16-22 weeks is the ideal time for them to be developed enough and not too big that it's hard to get a good view. And I've heard about some done at around 14 weeks and being wrong about the gender.