How young can premature ovarian failure occur?

Any age: It can happen at any age after puberty. It is not common though. And sometimes premature ovarian failure is temporary and the normal function may return.

Any Age: Some women never enter puberty. Some women enter menopause as a teenager and for some women it happens later on. So it can occur at any age. That's why it is important to be evaluated for irregular or absent periods, vaginal dryness, hot flashes, etc even at a young age.