It can persist into middle age, but this would be relatively unusual. Mature women often do have a low-grade grumbling acne on the chin, which actually can be quite difficult to treat. The hospital has left me to do my own research and let them know if I find any thing to help myself and others. There are several different types of porokeratosis. Unless you tell me which sort you have, I can't give the appropriate advice.
Prevention is better than cure.
When you have sunburn, the damage has already been done, so you need a soothing cream which cools from water evaporation. The topical antihistamines and anaesthetics may give some temporary relief. Dermatologists may also occasionally use a potent topical steroid. As a rule, one should expect to see no improvement for the first six weeks approximately of treatment, and then one may hope to see perhaps ten per cent improvement a month. However, we know now that some of the bacteria associated with acne are resistant to the antibiotics often used.