They actually start around 14 weeks, but everything is so small then that they're incredibly mild and therefore not noticed until much later. So anytime after that, it's totally normal for you to start feeling them.
Mine have been regular for a couple weeks now...they just feel like a sort of tightening...and they dont *hurt* they're just uncomforable...Its different than real contractions, in that a real contraction is like squeezing a stress ball, and you can actually feel (and sometimes see) your stomach..well...contracting lol. Braxton hicks feel like tightening, but I've never been able to tell when they start/stop, or feel my stomach contracting...I often feel like baby just sort of got lodged out in front when I get them
They're also supposed to ease up if you switch up what you're doing. So if they start up when you're sitting or laying down, if you get up n walk around a bit, they're supposed to ease up and vice versa....