What is the latest age that pubertal gynecomastia should resolve itself?

Gynecomastia: Benign pubertal gynecomastia may never resolve completely. If one started puberty late - it can resolve as late as 20 years old. Most cases resolve by the end of puberty - 16 to 17.

18: If it hasn't resolved by 18, then it probably won't.

1-3 Years Onset: Pubertal gynecomastia tends to spontaneously “regress” within one to 3 years after onset. Adult on-set gynecomastia will likely not regress unless it is associated with a condition such as obesity or caused by use of a specific medication or medical condition. Often best treated with partial excision of the prominent glandular tissue as well as liposuction surgery of the peripheral chest.