My son is 5, almost 6.
My Husband taught him Chess, at 4 years old.
Why? My SON... wanted to learn.
My Husband taught him.
He knows how. Not like an adult in sophistication of course, but he does know how. And he likes to play it.
Also he knows how to play Backgammon.
If your son is interested, the game "Go" is also fun.
Both my kids like that, and learned how, early on.
A child can learn anything. My son knows measurements, how to tell time, addition/subtraction, how to play those games I mentioned. He has an inclination... for these things. We know that as his parents. Thus, IF HE is interested in something, we teach him. Not based on his "age."