fertile-octogenarian rule

Definition from Nolo’s Plain-English Law Dictionary

An unrealistic legal fiction that any living person (male or female) is capable of having a child. In estate planning, this rule could defeat the intentions of a person leaving property to others. For example, if property could not pass to one's child as long as he or she might acquire a sibling, then the child would have to wait until both parents died, unnecessarily tying up the property. Most states have passed laws to cure this anomaly.

Definition provided by Nolo’s Plain-English Law Dictionary.