The DMG description of Potion of Longevity says:
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.
What does a 10% cumulative chance mean in this context?
Assuming that it means the chance increases by 10% each time, I find the statement awkward because usually when you state a cumulative probability, you are stating the total, not the amount that is added. That is, if we are to add 10% each time, then after 3 uses, there's actually a 20% cumulative probability that it ages you (not a 10% cumulative probability).
By a strict reading, I'd read that the probability is 10% each time after the first, but that is almost certainly not what was intended.