I started my (now 5 year old) son when he could support the helmet. We bought it when he was 5 months, got him used to the weight a little bit at a time, started with the trailer a couple of weeks later. Ours was pretty big so I had pillows to support his head when he fell asleep...he spent a lot of time sleeping in there at first.
I flipped the trailer twice, once on a path when I didn't quite clear a barrier and once on the road when it hit a very very small pothole on a turn. He remained suspended in the seatbelt. Not very happy with me but otherwise alright. A helmet's now second nature to him and he'll yell 'helmet' at you if you forget yours!

Another note. One of our friends went to a local shop for a helmet for their year-old daughter and what they came back with was completely unacceptable. The helmet was the smallest that shop happened to carry and instead of trying to find something suitable they sold it, saying there wasn't anything smaller. While this may have been true for the line of helmets they carry it doesn't happen to be true in general. It was simply too big and actually kept sliding onto the side of the little girl's head. DON'T let anyone talk you into something that doesn't fit.