Source: Dungeon Master's Guide

Potion, very rare

When you drink this potion, your physical age is reduced by 1d6 + 6 years, to a minimum of 13 years. Each time you subsequently drink a potion of longevity , there is a 10 percent cumulative chance that you instead age by 1d6 + 6 years. Suspended in this amber liquid are a scorpion's tail, an adder's fang, a dead spider, and a tiny heart that, against all reason, is still beating. These ingredients vanish when the potion is opened.