We got ours at 2 years. But I think we could have gotten it before. I brought it up at our clinic appt, they measured her and said, "Oh yeah, she's more than big enough." It was a tough transition, but after a week of struggle, it just became her daily routine.
Some good ways to transition were to put her favorite stuffed animal or doll in the vest. And the trick that really worked for us was to buy a new toy, (playdough worked for us), and then introducing it right as we put the vest on. So she associated playing with the vest. I think every kid is different, but that was our golden ticket. Good luck. -Elise