You actually have them the whole time, you just cant feel them untill later. Some never do feel them. I just started really being able to tell when I have them a few weeks ago and im 32 weeks.
With my first I started getting Braxton hicks around 20weeks wich is completely normal and with my 2nd I have had Braxton hicks since I was 13-14weeks which is also normal for already having a kid:) most wont get braxton hicks until around 19-24 weeks