Heirloom experience bonus work until the level they are designed (not including the level). So for WotLK heirlooms you will stop gaining the xp bonus once you hit 80 and for Cata (cloak and helm) it will stop once you hit 85.
The reason for this is to prevent people from using heirlooms to level through new content faster or forcing them to using heirlooms when a new expansion is released.
Also the amount of stats bonus that is on gear from new expansions will most likely make up for the lost xp bonus. Also Cloak and Helm do not gain Cata level of stats before level 81, so if you can find a green helm/cloak while being level 80 it will most likely server you better until you hit 81 and the heirloom gain the expansions stats.