Whether you are Intermediate and Advanced also varies from one type of exercise to another. I consider myself advanced in weight training and kickboxing, intermediate in step (I think, I don't know: I've been doing it for about 13 years, but I now use a 6" step--used to use 8"--and I don't do the high impact stuff, BUT I can "get" all of Cathe's choreography...but I tried Cristi Taylor once and couldn't/didn't want to get hers), low-intermediate (?) in yoga, beginner in pilates. Who know?
Collage video links these levels to amount of time one has been exercising, but as other posters have mentioned, you can exercise for a long time and never get into advanced workouts. Or maybe you are genetically gifted and CAN do advanced stuff pretty early on.
It's pretty easy to figure out what "beginner" is, because it's a workout that anyone could do with no background. But I think the other levels are sometimes very subjective.