[pbhjpbhj]

I first exposed my kids to binary, and other number bases, when they started learning the decimal place system (4 or 5 years). It seemed to fit really well there.

When they do/did multiplying by 10, and doubles, it's a good time for a review (why does a decimal shift produce a x10, etc.).

You can use that to talk about codes (crypto), communication (Morse, etc.), computation using "switches", logic. It's a good base for a lot of interesting stuff IMO.