When they want to. When they are interested they can probably handle it. Just tailor what you are doing to their needs.
My 6 and 10 year old play and have a great time. But we didn't pick it up until the 10 year old started asking some serious questions and stating he wanted to play.
This answer assumes that you currently roleplay and the kids see it is something you do (my kids are long used to roleplaying in the house). If you don't currently roleplay, and your kdiskids don't have any friends who currently do, then you may want to approach the matter differently than I did.