Hey! There’s no actual "minimum age" to play Tibia, and there’s no rule about a proper/permitted age either. However, CipSoft provides both a Parent’s guide to help parents understand the game and learn about online safety for children who play, as well as a Tibia Service Agreement that must be accepted before downloading the game. This agreement requires you to be either of legal age or to have your parents’ permission to play but it isn't really an impediment to playing.
Also, according to the ESRB rating, Tibia is rated Teen (14+), but this is only a suggestion based on its content, which includes fantasy violence, crude humor, and alcohol and tobacco use. So it pretty much leaves parents/kids the choice to play or not.