The life cycle of family size, at least in the U.S., has a very humped shape rather similar to that of income, though with a somewhat earlier peak. As a result, one might expect, and generally finds a fairly constant rate of saving in the central age group, but lower saving or even dissaving in the very young or old.
Source, in full
Franco Modigliani, "Life Cycle, Individual Thrift and the Wealth of Nations" Lecture to the memory of Alfred Nobel, December 9, 1985, in: Nobel Lectures, Economics 1981-1990, Editor Karl-Göran Mäler, World Scientific Publishing Co., Singapore, 1992.