Today in my eco-devo class we asked the question, "Why do we get old?". Student answers, a list:

• Time
• Methylation/acetylation
• Accumulation of damage
• Environmental damage
• IGF pathway
• Down regulation
• Mutations
• Stem cell depletion
• Competition
• Chance imperfections
• Invisible to selection

Always good to remind the young'uns of their inevitable fate.