Fertile octogenarian is a legal concept associated with property law which states that anyone can have a child at any time. This concept can impact whether or not children can be the sole beneficiaries of their parent's property before their death. Due to the fact that many people cannot have children due to age or disease, this concept is actually outdated and has been altered in many states.
Hypothetically, there could be a dispute about property ownership if an elderly couple is leaving property to an only child, but then they suddenly have another child. For example, if a couple in their early fifties has a will that states that their property will go to their descendants and they have a child right before they die, the newborn child could have property rights in addition to the original child. So, the term fertile octogenarian addresses this issue.