From Wikipedia:

Braxton Hicks contractions, also known as false labour or practice contractions are sporadic uterine contractions that usually start around 6 weeks however are not usually felt until the second trimester or third trimester of pregnancy

With my twin pregnancy, I was having them at about 28 weeks but I didn't even realize it until my doctor pointed it out to me. I have heard that for subsequent pregnancies you tend to recognize them alot earlier and can notice them early in your second trimester.

Shannon's answer matches what I've heard. However, if you're experiencing them, double check they really are Braxton Hicks. (Some things to look out for here). I went into preterm labor at 32 weeks without realizing it at first because the contractions were painless and I thought real contractions had to hurt, especially for the first child. I almost didn't call my doctor because I thought I was just being paranoid. Fortunately, everything turned out okay in the end and my baby was delivered at 40 weeks.

I start noticing minor contractions around week 32.

I didn't have any braxton hicks with either pregnancy or i didnt feel them. I just had very lively babies that squirmed alot.

I never really felt them or did not notice them. My baby was very active and would kick a lot.

Most people notice them more (and earlier) as they have more babies. A first time mom probably wouldn't notice them until 30 weeks or later. Funny story, though: my mother discovered that she was pregnant with her fourth child when she realized she was having braxton hicks contractions. She was only about 8 or 10 weeks along. :)