I don't have a final answer to this. However in m observation with different families and kids is that some start relatively early, before the kid can do arithmetics or such, they can count till a pile got enough money for the sweets or toy they want and then count the other pile to see how much is left and if they can buy the other.
Doing this in a digital system requires first some computing device (be it phone or laptop or whatever) then check the account value, read it correctly, then do some arithmetics and then interpret the result. That's a year or two older.
