Since starting my PhD, I have found out that my dept. only gives TA work to people on the 2nd/3rd years of their thesis, hence I have some teaching lined up for next year but am not doing it at the moment. I think the reason that some (not all) places are reluctant to give TA work to 1st years is not down to experience, but rather a desire to see you established in your research. TA work is supposed to be quite time consuming, and I can imagine that trying to get your thesis off the ground AND prepare, teach and mark work for a class, might be quite a challenge.