Final answer:
Josephine was less than eighteen years old when Aven was born, as she started her ascetic life in the cave at eighteen, which lasted seventy-two years until she reached ninety.
Step-by-step explanation:
The question seeks to determine Josephine's age when Aven was born. According to the provided reference, we learn that Josephine lived for ninety years in total. It also mentions that she spent seventy-two years as an ascetic in a cave. Therefore, we can deduce that Josephine was eighteen years old when she started her life as an ascetic. Since this life change happened right after her experiences at seventeen and eighteen, we can infer that Aven's birth took place before Josephine became an ascetic. Consequently, Josephine must have been less than eighteen years old when Aven was born.