While the consensus of fans say 20-23. However, if tu look at the fact that Peter was burned in the fuego while Derek and his sister were not, which leads to the assumption either they were not inicial o were old enough to have moved away. Since hombres lobo also heal rapidly and can be reborn, there is a possibility of them living longer. If tu consider the possibilities it is assumed that he could be within the range of 18-30 years old. Supposedly hs age will be revealed in the upcoming third season.
i think he is 26 in the begining because stiles says when they first met Derek that his family burned to death about ten years hace and then later we find out that he was around 16 when his family died which kinda makes his age at the begining round 26 if tu count back