A model for the probabilistic function followed in editing Wikipedia is presented and compared with simulations and real data. It is argued that the probability of editing is proportional to the editor's number of previous edits (preferential attachment), to the editor's fitness, and to an aging factor. Using these simple ingredients, it is possible to reproduce the results obtained for Wikipedia editing 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 edits per editor that seems to describe the editing behavior in Wikipedia.