When they are 12 – 13 years old, most children should be able to play adults board games. They need to put more maths skills to use, and use more complicated strategies while playing games. The level of complexity is high enough as this age. Monopoly and Chess are great for this ages. Basically, they should be able to play any educational board game that adults play as long as the educational content is not one that forbids children under 18 to be exposed to.