Multiplication flash cards start being useful when children are in third grade, or around 8 to 9 years old. This is when multiplication is taught in school.
In third grade, children start learning the fundamentals of multiplication, though some teachers might touch on it sooner. They learn basic fractions, decimals, multiplying and dividing. In addition to learning multiplication, students at this age also learn more complex adding and subtracting, as well as measuring height, weight and volume. There are printable flash cards, ones sold at stores and online flash cards. The online versions are convenient as they are used from a computer or mobile device.