The short answer is yes. In Pennsylvania one adult can adopt another adult. An adult adoption is the adoption of any individual who is at least 18 years old. Perhaps the more interesting and important question is, "why would someone want to adopt an adult?"
In Pennsylvania, as in every state, the purpose of drafting a will is to provide direction to surviving family members regarding what should happen to your personal and real property, and how to care for minor children of the testator. One of the most important aspects of any parent's will , is the appointment of a guardian of his or her minor child(ren). The appointment of a guardian ensures that the parent is able to select who is best able to provide for his or her child in the event that he or she becomes unable to.