A model for the probabilistic function followed in Wikipedia edition is presented and compared with simulations and real data. It is argued that the probability to edit is proportional to the editor's number of previous editions (preferential attachment), to the editor's fitness and to an ageing factor. Using these simple ingredients, it is possible to reproduce the results obtained for Wikipedia edition dynamics for a collection of single pages as well as the averaged results. Using a stochastic process framework, a recursive equation was obtained for the average of the number of editions per editor that seems to describe the editing behaviour in Wikipedia.