When does aging really begin? Two Northwestern University scientists now have a molecular clue. In a study of the transparent roundworm C. elegans, they found that adult cells abruptly begin their downhill slide when an animal reaches reproductive maturity.A genetic switch starts the aging process by turning off cell stress responses that protect the cell by keeping important proteins folded and functional. The switch is thrown by germline stem cells in early adulthood, after the animal starts to reproduce, ensuring its line will live on.