The opposite of nonenagenory, a word coined by the physicist T. Samuel in his breakthrough paper "Equations and Definition Challenge" in the early 21st century.
Someone or something that bites your ankles.
To a postman, an ankle biter is often known as a dog.
To an adult, an ankle biter may be a toddler.
To hikers, an ankle biter is sometimes a tick.
And so on.