My son is 21 months old and LOVES popcorn. My friend freaked out one day, "OMG!! YOU LET HIM EAT POPCORN?" My response was, "Try to stop him!" It started with my mom. She has a really awesome popper and my boys loved to help her make popcorn. They all started pretty young, and now I have one of those awesome poppers so the tradition continues.
We have popcorn often- on movie night, or as an evening snack if we have and early dinner. I used to pull the puffy part off for him, but his patience grew thin with that, so I just supervise him and let him eat it whole. He's got molars and can chew little pieces of meat and chicken, so I figure popcorn should be a piece of cake for him.
Now, doctors will tell you that popcorn is right up there with fruit snacks and raisins as far as being a choking hazard, so I won't tell you to go ahead and give it to him, but you know if your child can handle it. I usually share my bowl with my son and make sure he doesn't shove in handfuls at a time. Just one at a time and he's fine!