In English, an irregular plural is any plural noun that does not use an -s to indicate that it is plural.
The most common irregular plurals are:
These are more difficult to learn than regular plural nouns, and while English speaking children should start using them by 48 months, it make take until 60 months (5 years) of age before they use familiar irregular plurals consistently. What exactly constitutes a familiar word depends on the child's family, and may include common unchanging plurals (sheep, fish, deer, shrimp), or archaic and formal plurals (oxen, geese, dice, cacti), but it is quite common for even adults to have some confusion over some of these. Voicing changes (elf/elves, knife/knives) may take longer to emerge, and may be misarticulated (e.g., 'elves' becomes 'elfs') until around seven years of age, depending on how conscientious the parents are in correcting pronunciation.