Genetically identical individuals that grow in the same environment often show substantial phenotypic variation within populations of organisms as diverse as bacteria1, nematodes2, rodents3and humans4. With some exceptions5,6,7, the causes are poorly understood. Here we show that isogenicCaenorhabditis elegansnematodes vary in their size at hatching, speed of development, growth rate, starvation resistance, fecundity, and also in the rate of development of their germline relative to that of somatic tissues. We show that the primary cause of this variation is the age of an individual’s mother, with the progeny of young mothers exhibiting several phenotypic impairments. We identify age-dependent changes in the maternal provisioning of the lipoprotein complex vitellogenin to embryos as the molecular mechanism that underlies the variation in multiple traits throughout the life of an animal. The production of sub-optimal progeny by young mothers may reflect a trade-off between the competing fitness traits of a short generation time and the survival and fecundity of the progeny.