A study on the effects of prenatal exposure to smoke (by J. Nigg and N. Breslau, Journal of the American Academy of Child & Adolescent Psychiatry, vol. 46, 2009, pp. 362–369) indicated that mean birth weight was significantly lower for babies born to mothers who smoked during pregnancy. It also suggested that increasing age of the mother resulted in increased effects. Explain how this suggests interaction between smoking status and age of mother in their effects on birth weight of the child.