I personally wouldn't start them pin trading until 10 or so. Not because they won't like it, just because it's an expensive hobby! We get AP's periodically and go to the parks a lot during the year, so the year dd was turning 11 we wanted to do something different since she'd been to the parks so many times already. Pin trading was perfect, and her brothers liked watching. We'd no intention of them pin trade, and in fact told them when they were 10 like their sister they could pin trade, too. That is until we won YOMD lanyards! They traded those things right away, and of course 2 pins is not enough, so we wound up with all three kids pin trading. We got all of our pins from the Disney outlet for next to nothing, but still . . . anything X 3 adds up!
Dd loved trading and has many, many pins. Middle ds (6, turning 7 at the time) also liked it and it actually was a good thing for him--he's quite shy and learned to approach CM's and ask to trade. Little ds (3 turning 4) was too young. He liked it, but just because his siblings did.
So if you have a choice, don't start, especially if you are new to the parks. There is so much else to see and do!