Additional info: Wicket 1.5 will be supported until Wicket 7.0.3/4 according to a post from Martin Grigorov on Wicket's Mailing list
So a major release has 2 years life. Actually even more because 1.5.x will be supported until 7.0.3/4 before be moved to security maintenance mode. At least this has been the case with the previous releases.
So i think you have quite some time it will be supported in terms of secuirty.