I always considered the Mother Knytt in 'A Strange Dream' to be a separate person from Juni.
In terms of a Knytt's life expectancy, there are a few contradictory bits of evidence. Small, energetic mammals tend to live briefer lives than humans (a species' natural lifespan is roughly related to its heart rate; all mammals have roughly the same number of heartbeats in their lifetime, be they mouse or elephant - though humans have become an exception to this due to modern medicine. It probably takes a lot of energy and exertion to jump four times your own height). But sentience implies a longer lifespan (humans live about 20 years before reaching their full mental potential). And folkloric creatures tend to be very long lived (c.f. Tolkien's trolls, elves and dwarves, for example). So it's hard to know how to equate a knytt's lifespan to our own.
But as Chip said, Juni's stories are all folklore and legends even to her own kind. It is likely that her various stories actually happened to different people, or originated as pure fiction, and were amalgamated to be about a single hero, similar to how some people believe myths around figures like Thor developed.
In the end, Juni's age would depend on the story and the storyteller. In some she's young and naïve, in others more experienced, and in others she's near the end of her life (to quote from KU, 'As the story goes, one day Juni found the last wild snowdrop. She took it home, planted it by her window and named it the end flower. Later that day Juni died.')