My oldest has probably figured out ballpark figures. I think as they get older and close to choosing careers and moving on their own, they really need to know. The life they get to live under their parents roof (usually best years in terms of income) does not reflect life as an entry level worker at 22. I think it's beneficial to know it costs X for this standard of living, and when you start out, you will only make Y which affords a smaller standard of living. Btw I learned my parents income when they filled out the FAFSA for college. I was shocked they earned so little and yet we always felt "fine"