Actually all of the pros the except Janette were competition dancers, since they were children. All the titles AJ went on about were underage titles, he never actually danced at competition level as an adult. Anton only danced competition level for a few years as an adult too.
Most of the pros left competition dancing for Burn the Floor in their 20's though. The only pros who haven't been in BTF are Neil & Katya, Oti, AJ, Amy & Anton. The BTF dancers though are far more aware of themselves & also when seeing any of their shows they steal the stage, as they have had years if practice on stage. It's why Dianne, Karen, Gorka & Giovanni stood out so much on the pro tour, even though Oti, Neil & Katya are the best dancers technically, they don't have the years of stage experience of some of the pros.
As for starting at a young age, Amy only started dancing around the age of 12 it's the same with AJ, so I think they are just oblivious to how they come across on camera, but it has nothing to do with them dancing at a young age.