I think this is like the question "What age is too young to bring a child to WDW?" Everyone is going to have their own opinion. We started pin trading when my youngest was five. On our last trip we did not even bring pins with us to the park. I think part of it for me was it was becoming too much work to keep track of which pins I had and which pins I needed to complete sets. (So frustrating when you think you traded to complete a set only to get back to the hotel and realize you already had that one.)