I noticed yesterday that my sim, who had gotten a tattoo at the salon, was 4 days younger than he should have been. I sent him to get another one, and sure enough, 4 more days had been deducted from his age. This is probably a bug, but follow my thought: maybe getting a tattoo makes a sim feel younger, and therefore reduces his age. Maybe it explains the age-defying members of Motley Crue...