Just found this on the bottom of a recently rooted plant. By its appearance it could be either iron or magnesium, but by its position on the plant (it was at the bottom), it is magnesium, which is mobile, meaning the plant can translocate the nutrient. What happens is that the plant runs low on magnesium and so moves it from older leaves to new growing shoots, leaving the older leaves to show symptoms. Note the red petiole, which always precedes the yellowing. The easy fix is to boost the micro if in hydro, or add 1 tsp/gallon of epsom salts to your water for one or two waterings if in soil. A foliar application of the same will speed recovery in either medium.